The use of activity theory is considered in the evaluation of a web based academic writing course in a New Zealand university. Activity theory is an aspect of sociocultural theory and provides a model for the understanding of goal directed social activity. Like other recent developments in applied linguistics, research and evaluation in second language writing has been influenced by sociocultural theory, because it emphasises the social, rather than the individual, context of writing. The primary purpose in carrying out this study was to illuminate the use of activity theory as a formative evaluation technique for the improvement of large academic writing courses supported through the web conferencing features of a course management system, "Web Crossing". Data were in the form of international student responses to prompts made in online diaries on a weekly basis throughout the course. An activity theory orientation guided the design of the prompts and the analysis of the data. Activity analysis allowed the researchers to appreciate the tensions and difficulties for students in managing the group processes that the web mediated instruction afforded or constrained. The study also suggests that the set of guiding questions derived from Jonassen and Rohrer-Murphy (1999) may be useful for future evaluations and research. (Contains 1 endnote.)