The present paper examines lexicon organization and lexical uniqueness through a lexical availability task. Previous research has concentrated on exploring via word association tests how learners organize their L1 and L2 lexicons. Additionally, the closeness between the native and the L2 lexicons are also object of analysis in research. Lexical uniqueness has also been used as a measure to determine “nativeness”. In the present study, we had two groups of Greek B1 and C1 level learners of Spanish FL answer a lexical availability task and compared their results with those of a group of native speakers. We found that proficiency level is crucial in the determination of lexical uniqueness and lexicon organization via lexical associations obtained with a lexical availability task. Furthermore, our results revealed that thematic field is a relevant factor in speakers’ associative behaviour and lexicon organization. Results are discussed in light of previous research findings and pedagogical implications are proposed.