Bilingual learners' inadequate English vocabulary size is one of the most frequently stated problems in applied linguistics. Using mostly qualitative case studies, earlier findings reported several factors (e.g . negative attitude, lack of exposure, poor proficiency, and a ge of a cquisition) as related to bilingual learners’ insufficient English vocabulary size. Therefore, the present study undertakes an in-depth quantitative study to measure language dominance, which encompasses language history, use, proficiency, and attitude towards Malay, the f irst l anguage (L1), and English, the second languages of 96 Malay-English bilingual undergraduate s . It also examines the relationship between vocabulary size and overall dominance score s . The 20,000 Vocabulary Size Test was used to measure English vocabulary size and the Bilingual Language Profile (BLP ) was used to determine the dominance score. The r esults show that most of the Malay-English bilinguals are Malay dominant. A positive and statistically significant association was found between the overall language dominance score and English vocabulary size. O ne of the most striking findings to emerge from this study is that the language dominance score accounts for 30% of the variability in the vocabulary size of Malay-English undergraduate s . Keywords: v ocabulary size; b ilingualism; language dominance; s econd l anguage a cquisition; tertiary students; language testing