Self-reporting procedures and inspection methods have been largely employed in the fields of interaction and web-design for assessing the usability of interfaces. However, there seems to be a propensity to ignore features related to end-users or the context of application during the usability assessment procedure. This research proposes the adoption of the construct of mental workload as an additional aid to inform interaction and web-design. A user-study has been performed in the context of human-web interaction. The main objective was to explore the relationship between the perception of usability of the interfaces of three popular web-sites and the mental workload imposed on end-users by a set of typical tasks executed over them. Usability scores computed employing the System Usability Scale were compared and related to the mental workload scores obtained employing the NASA Task Load Index and the Workload Profile self-reporting assessment procedures. Findings advise that perception of usability and subjective assessment of mental workload are two independent, not fully overlapping constructs. They measure two different aspects of the human-system interaction. This distinction enabled the demonstration of how these two constructs cab be jointly employed to better explain objective performance of end-users, a dimension of user experience, and informing interaction and web-design.