This mixed methods study explores EFL students’ experiences and perceptions as they learn to write a composition with ChatGPT’s support in a classroom instructional context. Students’ perceptions are explored in terms of their motivation to learn about ChatGPT, cognitive load and satisfaction with the learning process. In a workshop format, twenty-one Hong Kong secondary school students were introduced to ChatGPT, learned prompt engineering skills, and attempted a 500-word English language writing task with ChatGPT’s support. Data collected included a pre-workshop motivation questionnaire, think-aloud protocols during the writing task, and a post-workshop questionnaire on motivation, cognitive load, and satisfaction. Results revealed no significant difference in students’ motivation before and after the workshop, but mean motivation scores increased slightly. Students reported high cognitive load during the writing task, especially during prompt engineering. However, students expressed high satisfaction with the workshop overall. Findings indicate ChatGPT’s potential to engage EFL students in the writing classroom, but its use can impose heavy cognitive demands. To ensure that ChatGPT use supports EFL writing without overwhelming students, educators should consider an iterative design process for activities and instructional materials and carefully scaffolding instruction, especially for prompt engineering. Copyright © 2024 The Author(s).