Critical thinking is believed to be an essential skill for 21st century survival and therefore has been widely promoted in education. In Hong Kong, critical thinking is one of nine generic skills to be developed across all subjects, including English. How students do critical thinking in ESL, which is seldom used outside school and yet holds high social value, has, however, been underresearched. This article is concerned with how some low-English-proficiency senior secondary students in Hong Kong conducted critical talk in English. The study specifically investigates how the students used English to express ideas that were first developed in Cantonese (the students' first language). Based on a discourse analysis of the criticality and elaborateness of the Cantonese and English utterances of one group of students, the authors discuss findings that reveal a significant contrast between the students' more elaborated discourse in Cantonese and a restricted discourse in English characterised by reduced content and limited lexicogrammatical structures. The findings call for more attention to the impacts of linguistic proficiencies on critical thinking performance of ESL learners and to how the communicative gaps in critical literate talk revealed in ESL learners' first and second languages can be gradually reduced.