The purpose of this study was to examine the effects of volunteer assistance at the 2005 Special Olympics World Winter Games (SO) in Nagano, on Japanese university students' motivation to learn English. The construct of motivation was investigated within the fraework of self-determination theory, which assumes three basic psychological needs: for competence, for relatedness, and for autonomy. According to the degree to which these psychological needs are satisfied, social-contextual factors are considered to facilitate or impede motivation. The theory posits that in terms of the degree of self-determination or autonomy, motivation is categorized as (a) amotivation, (b) extrinsic motivation, and (c) intrinsic. Extrinsic motivation is further divided into (a) external regulation, (b) introjected regulation, (c) identified regulation, and (d) integrated regulation; in this order, the degree of self-determination increases. Previous studies on motivation to study English as a second language (L2) within the framework of self-determination theory (Hiromori, 2003a, 2006; Hiromori & Tanaka, 2006) have shown that L2 learners’motivation to study may change and that social-contextual factors facilitative for basic psychological needs may influence changes in motivation. The research questions posed for this study were: Did participation in the SO as volunteers cause changes in Japanese university students’motivation to study English? and (b) if so, how? We supposed that such participation would provide the students with opportunities through which their basic psychological needs might be met. Thus, it was hypothesized that (a) participation in the SO would lead to changes in motivation to study English and that (b) the motivational types with higher self-determination (e.g., intrinsic motivation) would be enhanced and the motivational types with lower self-determination (e.g., amotivation and external regulation) would be diminished motivation. A 16-item questionnaire was administered twice: once before the event (survey 1) and then two months after the event (survey 2). Responses from 44 students were analyzed. Because of deviation from the normal distribution for the scores for amotivation, non-parametric tests (Wilcoxon signed-rank tests and Friedman’s tests) were performed. Descriptive statistics showed that the median scores of amotivation were the lowest for both survey 1 and survey 2, whereas the median scores of identified regulation were the highest and that the median scores of external regulation and introjected regulation decreased within the two-month period, whereas the median scores of amotivation, identified regulation, and intrinsic motivation increased. First, Friedman’s tests and the post-hoc tests revealed that, before participating in the SO, amotivation was significantly lower than any other type of motivation, while identified regulation was significantly higher than any other type of motivation. There were no significant differences among the other combinations of motivational types on survey 1. After two months, changes were observed: since the median scores of amotivation and intrinsic motivation increased and the median score of introjected regulation decreased, the significant difference found between amotivation and introjected regulation on survey 1 disappeared on survey 2, and the difference between introjected regulation and intrinsic motivation became significant on survey 2. Wilcoxon signed-rank tests revealed that the change in the median scores of intrinsic motivation between survey 1 and survey 2 was significant. In summary, the results confirmed the hypotheses. The discussion of the results is structured around the following three points. First, the findings support the arguments made by Hayami (1995) and Horino and Ichikawa (1997) that motivation should be treated as a dynamic changeable cognitive state. At the same time, considering the two-month interval after the SO, the changes of motivation observed in this study may not be temporal, but durable. Second, the results suggest that participating in international events like the SO may be facilitative for the improvement of motivation even if the events are short. Most of the participants in this study reported that they had assisted at the SO for three to five days (n = 40, 90.9%). Third, it is pointed out that satisfaction of the need for competence alone may not influence motivational changes because most (n = 28, 63.6%) reported that they had spent a total of two hours or less speaking English during the volunteer work. Thus, the participants may not have had sufficient experience in communicating with others in English. Finally, several limitations are discussed: the sample characteristics, the design without any control groups, the questionnaire with a small number of items for each type of motivation, and the unresolved question as to how participation in the SO influenced motivation. Future research is needed to overcome these limitations and thereby to obtain insight into how participation in international events should be incorporated into language programs. 本研究の目的は、2005年スペシャル・オリンピックス(SO)冬季世界大会・長野にボランティアとして参加したことが、日本語話者大学生の英語学習の動機づけにどのような影響を及ぼしたのかを調べることである。動機づけの理論の1つである自己決定理論を援用して作成した質問票をSOの直前と2ヶ月後に実施した。SOにボランティアとして参加した大学生44名の回答を分析対象とした。結果は次の通りである。外的調整と取り入れ的調整の中央値は、事後調査において下がり、一方、無動機、同一視的調整、内発的動機の中央値は事後調査において上がった。これらの変化のうち、統計的に有意であったのは内発的動機であった。すなわち、SOのような国際的イベントへのボランティアとしての参加が、日本人大学生の英語学習動機に影響を及ぼしていることが確認された。