School- and university-based writing tasks make demands on cognitive processing and socio-cultural knowledge that vary with the discipline in which the writing is situated. The variation in socio-cognitive demands makes the use of self-accessed, computer-mediated writing instruction more promising than conventional, teacher-fronted instruction. To be effective, self-accessed writing instruction should be constructed in partnership with student writers whose composing problems would form the basis of the content and organization of the instructional package. This paper demonstrates how a socio-cognitive model of writing informs the development of "Essay Assist", a computer programme for guiding students towards appropriate decisions in the essay writing process. Results of a preliminary trial are reported and their implications discussed. (Contains 2 tables and 4 figures.)