The following article reports the results of an action research project conducted in a public university in Chile. The project consisted of exposing ten undergraduate students from an English pedagogy program to a genre-based approach to writing expository essays. During eight weeks the three stages of the genre-based approach, namely: deconstruction, joint construction, and individual construction, were adapted and developed with the participants. Results gave evidence that a genre-based approach to teaching expository essays is effective in this context because after the application of the action research project, the students' compositions improved in comparison to the participants' pre-intervention essays.