Background: This study compared two tests of second language (L2) depth of vocabulary knowledge, namely the word association test (WAT) and vocabulary knowledge scale (VKS), with respect to their associations with vocabulary size. The same relationships were further examined separately for the five word-frequency bands of the vocabulary size test. Method: 115 Iranian English as a Foreign Language (EFL) learners who were native speakers of Persian took the WAT, VKS, and Vocabulary Levels Test (VLT). The selected participants were undergraduates who ranged from freshmen to junior and were both male (n=47) and female (n=68) students. Results: The outcomes of multiple linear regression analyses indicated that: (a) while both measures of vocabulary depth were predictive of the VLT, the WAT had a higher association with the dependent variable; (b) both the WAT and VKS were predictive of the high-frequency vocabulary, with the relationships being more significant for the WAT; (c) the WAT could significantly predict the mid-frequency vocabulary, whereas the VKS had no significant contribution; and (d) while the VKS was significantly associated with the low-frequency vocabulary, the WAT had no significant contribution to the prediction of this level. Implication: The implications of the findings are interpreted with reference to the suitability of both the WAT and VKS depending on the type of input, expected response, and desired frequency of the target words.
