Emotional appeal is a key dimension in user experience that often goes unmeasured in most user-centered design projects. This paper presents preliminary work for developing a set of guidelines for efficiently, easily and cost-effectively assessing the users’ affective state by evaluating their expressive reactions during an interface evaluation process. The evaluation of this dimension complements the analysis of the objective and quantitative data gathered through usability tests and the subjective feedback provided through post-test questionnaires. Categories and Subject Descriptors H.5.2 [Information interfaces and presentation] User Interfaces General Terms Measurement, Documentation, Design, Experimentation, Theory. Keywords Affective computing, human-computer interaction, evaluation, usability testing, emotions, heuristics, user-centered design.