The study focused on adult second language (L2) learners structure of the mental lexicon and the dimensions on which the overall state of their vocabulary knowledge can be examined. A model of studying lexical competence was proposed, which distinguished within the receptive and the productive domain two levels of assessment analysis: a microlevel, knowledge of the grammar of individual words, and macrolevel, the overall state of a learners lexicon as reflected by vocabulary size, quality of lexical knowledge, and metacognitive awareness. The experiment designed to test the proposed dimensions was an attempt to conduct a model-based research that explored lexical competence of adult native speakers (NS) of English, L2 advanced, and intermediate learners with regard to their language proficiency and varying degree of familiarity with words. The quantitative analyses confirmed that the intermediate learners differed significantly from the other two groups in all factors used in the study but metacognitive awareness, whereas the advanced learners overall state of the lexicon closely resembled that of NS across all variables but one nativelike typicality of associations. An in-depth analysis of this variable revealed that just as most NS maintain common word association networks, L2 learners also have stable patterns of commonality of associations that do not, however, resemble the ones of NS. It was also argued that a more constructive way of studying how speakers of two or more languages organize their mental lexicon would be to consider the stability of their meaning connections rather than the degree of nativelike commonality of their word associations. Next, by using several methods of regression analysis, a smaller set of factors that reflected the participants language proficiency was identified as reliable predictors of lexical knowledge for assessment purposes. Similarly, a model comprising of the best set of predictors was proposed, which was as practically efficient as the full model. Finally, the interpretation of the lexical relations between the word associations and the stimulus words used in the experiment complemented meaningfully the overall qualitative and quantitative examination of the differences between the way NS and non- NS organize their mental lexicon.