Ability to write focused expository essays in English is the key to academic success for students in an English-medium education system, but such writing can be a challenge for students for whom English is a Second Language. Focusing writing lessons on improving topic content and grammatical accuracy does not always achieve the expected results. Proceeding from the theoretical view of writing as simultaneously social activity and thinking process, this study sought to determine if explicit teaching of the expository essay genre practices and the thinking processes underlying these practices would improve the quality of students' essays. Instruction featured deconstruction of sample texts and exercises to practise specific thinking strategies and grammar for realising the desired genre practices. Significant improvements were found in students' stance support moves and functionality of topics in post-instruction essays. The discussion section argues that these findings point to the effectiveness of a social-cognitive approach to teaching writing. (Contains 3 tables.)