This interlanguage study examines the L2 mental lexicon of Thai EFL learners with different degrees of language exposure--i.e., the high exposure group and the low exposure group. The scores from the English Language Exposure (ELE) Questionnaire were used to select the two groups of participants. To explore the lexical processing and the organization of the mental lexicon, two psycholinguistic tasks were employed: lexical decision task (LDT) and word association task (WAT). The LDT was used to investigate the semantic priming effect of the prime on the target words which in this study are the frequently co-occurring words or the lexical collocations of the verb + noun in the Thai learners of English with high and low exposure to English. The results exhibit that the collocational processing is faster than the noncollocation. However, the difference is not outstanding. The WAT (McNeill, 1966) was conducted to investigate the organization of L2 learner's mental lexicon or the association between the words prompted and their networks. The findings indicate that the L2 mental lexicon is mostly meaning-based. The majority of links between words in the mental lexicon engages meaning and concept. The results support the interlanguage phenomenon that the two groups of participants have different mental lexicon and different paths of lexical access. The learners with high degree of language exposure tend to have the stronger links between English words in the lexicon than the learners with low exposure. The high-exposure learners produce more meaningful responses than the low-exposure learners do. The production of phrases and chunks by the high-exposure learners is considered a characteristic of native speakers. On the other hand, the production of the low exposure group exhibits the L1 transfer found common in EFL learners.