Physiological indicators of arousal have long been known to be sensitive to mental events such as positive and negative emotion, changes in attention and changes in workload. It has therefore been suggested that human physiology might be of use in the evaluation of software usability. To this, there are two main approaches or paradigms: (i) comparisons of physiological readings across periods of time to indicate different arousal levels under different circumstances, and (ii) the detection of short-term (occurring in seconds) physiological changes in response to specific events. Both approaches involve methodological, analytical and interpretational difficulties. Also, the tight experimental controls usually adopted in psychophysiological experimentation can be at odds with the needs of applied usability testing. This paper reports initial investigations of these approaches and difficulties in the evaluation of software interfaces. From exploratory data, a preliminary model is proposed which combines the two paradigms for identifying significant HCI events. Explorations of the model within the context of a web-related task are then discussed. These explorations suggest techniques and procedures for applied usability testing, and the results point to ways in which physiological data may be informative about software usability. However, further investigations involving variations in task and procedure are required.