This study aims to determine the possible relationships among foreign language vocabulary size, foreign language grammar proficiency, and critical thinking skills. To this end, 126 adult language learners were examined in terms of vocabulary size, grammar proficiency, and critical thinking skills. The results revealed that participants' grammar proficiency levels and their vocabulary size scores correlated significantly with certain aspects of critical thinking skills. However, unlike their vocabulary size scores, grammar proficiency level scores of the participants correlated significantly with their overall critical thinking skills scores. These results will be discussed in relation to adult language learners.