Critical thinking, as a highly valued generic skill, lies at the heart of education worldwide. Although research suggests a close relationship between language and thinking, the integration of critical thinking into second language acquisition is under-investigated. This study therefore explores the viability of exploratory talk and WebQuests for critical-thinking development in learners of English as a second language. The study constitutes an exploratory cycle of a 'design research' project conducted in a primary school in Hong Kong. The project as a whole collected data from five different sources (i.e. tests of critical thinking, audio-recordings of group discussions, classroom observations, students' written work, and interviews with teachers) and had 125 Primary Five student participants (aged 11-12) and five English teacher participants. Drawing on both quantitative and qualitative findings on classroom dialogue, this study reveals the participating students to have displayed critical thinking through the use of explicit reasoning and exploratory talk in group discussions. The findings have pedagogical implications, offer suggestions for critical-thinking cultivation in the English-as-a-second-language context and contribute to scholarly understanding of the need to integrate instructional strategies for critical thinking into language classrooms. Copyright ©2021 Elsevier .