A study examined the differences between the processes involved in analytic and summary writing by contrasting student writing of both kinds. Twenty 11th graders, 10 high and 10 average ability writers, participated in two composing-aloud sessions. In one session, the student wrote an analytic essay about a history passage and, in the other session, a summary of a different history passage. Think-aloud protocols were divided into individual communication units, each of which was analyzed for seven categories representing underlying features of the writing process. In analytic writing, students used a more varied and complex set of thinking operations than when writing summaries, for which they mainly restated pieces of content from the reading, with little interpretation of content. Overall, results indicate that, where critical thinking skills are concerned, the nature of the writing task makes a significant difference, and that the act of writing can serve as a powerful heuristic--a tool for thinking critically about subject matter. (Author/EL)