Grammaticality judgments are a metalinguistic task type frequently used in second language research. With the addition of response time measurement, often called reaction time measurement, a measure of metalinguistic performance is achieved that can be used to investigate the degree of automatization of linguistic knowledge in language users. This study examined the speed and accuracy of grammatical acceptability judgments of English second language (L2) and Finnish first language (L1) sentences by university students of English (n=66), and the relationship of these judgments to English proficiency and foreign language communicative anxiety. After cluster analysis the students were divided into four groups: those who had a high English proficiency, those who were accurate, or slow in their English judgments, and those who had a lower English proficiency and who were also inaccurate in their judgments. It is suggested that one reason for the systematic differences in performance could be students' language learning history, as revealed in the background questionnaire, which might have led them to structure their language proficiency differently. It also seems that students may have differences in the language-independent ability to solve metalinguistic problems such as these. (Author/VWL)