Due to the undeniable role of education in raising environmental awareness, and as an effort to promote environmental literacy in ELT, this study sought to investigate the integration of environmental themes into English language classes to probe into its impact on EFL learners’ understanding and perception of environmental issues, and their motivation to act environmentally responsibly, and to shed light on any perceived differences between L1-based and L2-based environmental literacy instruction. To this end, twelve female English learners who were selected through convenience sampling participated in content-driven English sessions on environmental issues and recorded their diaries after each session of instruction. To analyze the qualitative data from the diary entries, bottom-up data-driven thematic analysis was used in order to extract recurring ideas, viewpoints, experiences, and reflections. The results of open coding showed that the participants’ understanding and awareness of environmental issues and their motivation to act responsibly to environment partially increased as a result of participating in environmentally focused English sessions. Although some participants declared no difference or prioritized mother tongue instruction for environmental literacy, a majority of them perceived differences in their experiences of learning about environmental issues in English versus their mother tongue, identifying perceived prestige of the language, unfiltered access to information, and association with a worldwide perspective among other factors, which made the English content more persuasive for them. The implications highlight that integrating environmental themes into ELT can promote environmental awareness.