This paper reports on the "usability" element of a study examining the development of a multimedia Learning Environment (LE) for Special Educational Needs (SEN) students, designed to match learner requirements to learning materials, as elicited from cognitive and accessibility profiles drawn up by teachers and learning support assistants. In developing the system, the researchers examined how usability tests could be undertaken that meaningfully involved people with cognitive disabilities. Methods were developed following interviews with staff and much observation of lessons involving the use of computers. The usability tests themselves consisted of a series of graded activities drawn from various SEN sources. They required only one or two actions--activating a mouse and/or guiding a cursor--for completion. The study highlighted a number of issues inherent in working with a user group comprising SEN students, including interpreting non-compliance and other behaviour patterns, making tasks meaningful, and accounting for the presence and degree of assistance given by a helper. Of particular interest was the issue of matching the intellectual level of tasks with the physical ease-of-use of the system. The paper suggests various methods for obviating problems elicited, and makes recommendations for the further development of computer-based activities designed to promote independent learning and inclusivity.