CONTENT ANALYSIS of three popular science textbook series for grades 4, 5, and 6 generated three types of unfamiliar words: Technical, Technical Support, and NonTechnical. The analysis was used to (a) test Finn's (1978) observation of a curvilinear relationship between Text Frequency and Standard Frequency Index, a major issue in his case for the transfer feature theory; (b) analyze interactions among Text Frequency and readability level relative to the three types of unfamiliar words, a further test of the transfer feature theory; and (c) reconsider effects of unfamiliar words on readability indices of science textbooks. The results support Finn's observation of curvilinearity between Text Frequency and Standard Frequency Index. As a result, the study concludes that any readability formula ignoring Text Frequency of unfamiliar words inflates readability index scores for science passages in which Technical words are repeated regularly. Variability among the three types of unfamiliar words was striking across various science content areas. Average sentence length contributed little to inflate readability levels, but counting unfamiliar words without considering Text Frequency and Standard Frequency Index and without discriminating Technical from other types of unfamiliar words is an unreliable indicator of a science text's readability, because it ignores the transfer features of Technical words.