This dissertation investigated Taiwanese English as Foreign Language (EFL) learners' incidental noticing and their subsequent language learning in relation to learner proficiency level and dyadic type in a text-based computer-mediated communication (CMC) environment. Sixty participants were included to form 30 dyads. At random, eight low-intermediate and eight advanced nonnative English speakers (NNESs) were paired with 16 native English speakers (NESs) to form 16 NES-NNES dyads; another 14 advanced NNESs and 14 low-intermediate NNESs were paired to form 14 mixedproficiency NNES-NNES dyads. The results revealed that the synchronous computer-mediated communication (SCMC) medium could, in general, enhance the occurrence of learners' incidental noticing and their subsequent second language (L2) learning regardless of learners' proficiency levels and dyadic types. No significant differences were found in the amount of the language-related episodes (LREs) produced by the NES-NNES dyads when compared to the NNES-NNES dyads. With regard to the number of LREs generated by the learners of different proficiency levels, the results showed that: (1) in the NESNNES dyads, no significant difference was found between the low-intermediate and advanced learners, and (2) in the NNES-NNES dyads, the low-intermediate learners produced a significantly greater number of LREs than their advanced interlocutors. In terms of the effect of interlocutors' proficiency levels on the number of LREs produced by the learners, the results revealed that: (1) the low-intermediate learners in the NESNNES dyads produced a significantly greater number of LREs than the low-intermediate learners in the NNES-NNES dyads, and (2) the advanced learners in the NES-NNES dyads also produced a significantly greater number of LREs than the advanced learners in the NNES-NNES dyads. With respect to the learners' performance on both posttests, the results of chi-square analyses showed that: (1) no significant differences were found both within and across the two dyadic types, and (2) no significant differences were found between learners of different proficiency levels within and across both NES-NNS and NNES-NNES dyads. Logistic regression analyses revealed that five LRE characteristics (type, source, complexity, proficiency, and successful uptake) in the NES-NNES dyads and three LRE characteristics (proficiency, timing and successful uptake) in the NNES-NNES dyads were shown to be significant predictor variables of the learners' subsequent L2 learning. Successful uptake was the most prevalent predictor variable of the learners' subsequent L2 learning across the two dyadic types. Besides, proficiency appeared to be the second prevalent variable but played a different role in these two dyadic types. Considering the language aspects focused in the LREs, negotiations on the linguistic features of grammar, vocabulary, and spelling were much more prevalent than the pragmatic aspects of language.