‘Pen1’ and ‘khʉʉ1’ in Thai have traditionally been regarded as copular verbs comparable to ‘be’ in English. Appearing in a copulative sentence, the two Thai copula verbs, however, differ in polarity-sensitivity. The present study demonstrates that the difference in polarity-sensitivity of the two Thai copulas cannot be accounted for within the theory of polarity-sensitive items previously proposed. Investigating the aspectual properties of the two Thai copulas in comparison with those of English copula, this study suggests that an explanation for the difference in polarity-sensitivity of the two Thai copulas might involve their aspectual properties. Contributing to the study of aspect and polarity-sensitivity, the present study reveals differences between Thai and English copulas and provides additional support for the idea that the macro-category of so-called copular verbs is too vague to describe cross-linguistic variation.