The current study examines the perceived fairness of an educational assessment process, considering the influence of positive and negative affect. The first objective was to determine if a person's evaluation of fairness fluctuates depending on the incidental affect (pre-evaluation affect). The second objective was studying the connection between emotional reactions (post-evaluation affect) and the perceived fairness of the assessment. Finally, we verify if the change in affect between pre-evaluation and post-evaluation is mediated by perceived fairness. A total of 607 students reported their evaluation of perceived fairness as well as their measured affect after and before the same exam. Results suggests the need to control for the impact of negative affect when investigating variations in the perception of fairness, as well as the need to take preventive actions to promote a positive affective state before exams.