L2 (second or foreign language) research indicates that vocabulary knowledge is not only the “single best predictor of text comprehension,” but also a strong indicator of listening, speaking, and writing proficiency (Alderson, 2000, p. 35). Understanding the development of vocabulary knowledge, including both vocabulary size and vocabulary depth, or quality of vocabulary knowledge—is therefore essential to the building of an overall insight into L2 proficiency. This study aims to explore the developmental status of vocabulary depth among postsecondary CFL (Chinese as a foreign language) learners of higher proficiency levels who have studied Chinese for over four years. In particular, it focuses on these learners' identification of two types of word association—synonym and collocational associations and how factors such as association type and target-word frequency impact association identification. The process and strategy use that are involved in the inference of word association are also explored. For these purposes, this study employs a combination of quantitative and qualitative research methods. Cross-sectional data were collected through a paper-and-pencil test of Chinese word associates from seventeen learners within five postsecondary CFL programs in the U.S. Each participant worked on two test booklets assessing synonym and collocational associates respectively for the same 44 adjectives selected from the three word frequency levels of below 1000, between 1000 and 5000, and above 5000. A two-factor within-subjects ANOVA revealed both significant main effects for association type and word frequency on association identification and a significant interaction between the two. Simple effect analysis and pair-wise comparisons further revealed that association identification became increasingly stronger with the increase of word frequency for collocational association, yet remained non-impacted by frequency before reaching the mid- to high-frequency transition for synonym association. Meanwhile, CFL learners' collocational knowledge was significantly higher than synonym knowledge at mainly the medium- and high-frequency levels. These reslts indicate that synonym knowledge seemed to lag behind in development as familiarity with words increased, but began to catch up at higher-frequency levels. Interview data collected from six CFL learners show that they employed a wide variety of knowledge sources, such as radical knowledge, morphological knowledge, contextual clues, sound information, or L1 in inferring word association. Inference success seemed to be influenced not only by their preexisting word knowledge, but also an integrated and flexible use of linguistic and contextual information in the inference process. Implications of these findings are discussed in relevance to curriculum and pedagogical development of CFL teaching and the understanding and definition of CFL proficiency in general. This study fills a gap in CFL vocabulary research by building a tentative measure of vocabulary depth and bringing greater insights into the developmental status of higher-level CFL learners in synonym and collocational association as well as the process that is involved in inference of word association.