The Delivering Inclusive Access to Disabled and Elderly Members of the community (DIADEM) project is funded through the Framework 6 European Union (EU) research programme. Its aim is to develop the DIADEM application which personalises the online form interface according to individual users’ needs, making the content more accessible for cognitively impaired older adults. In this paper, we present a trial protocol which has been specifically designed to identify cognitively impaired older adults, and to evaluate the usability of online form content from an older adult user’s perspective. To demonstrate the applicability of the trial protocol within the context of an ongoing research project, details of pan-European trials involving 77 eligible users, who evaluated DIADEM enabled online forms according to the trial protocol, are presented. The outcomes of the trials reveal a number of online form design recommendations, which will be incorporated into future versions of the DIADEM application. Although these have been primarily developed for informing the design of future versions of the DIADEM application, they also provide a valuable point of reference for online form developers more generally, which if followed, will ensure that content is more usable for the cognitively impaired older adult user group. To conclude, the lessons learned from implementing the trail protocol, and its appropriateness for this target user group is discussed, as well as how the findings may be incorporated into future versions of the DIADEM application.