Earlier publications have shown that a Heart Period Variability (HPV) -based methodology, after careful adaptation, could be a powerful technique for monitoring mental effort in Human-Computer Interaction. This paper outlines the INTERFACE testing workstation developed by researchers of our department. This system is based on the simultaneous assessment of HPV, time data of keystroke and mouse events, video images of users’ behaviour and screen content, etc. It is capable of identifying quality attributes of software elements with a time-resolution of only a few seconds. Our series of experiments demonstrate the practical usability of this improved methodology for testing user interfaces. The method of analysis allows us to decide what types of problems are significant to the users, and what types of problems set back the users only slightly. On the other hand, the method allows us to decide, to what extent the found problems and their assessed severity concern all the users in general, or how these things depend on the type and characteristics of the users. At the end of this paper, we will give a brief description of the further development of this INTERFACE methodology: we are in the process of integrating also another physiological channel – Skin Conductance (SC).