A longitudinal field experiment was carried out over a period of two weeks to examine the influence of product aesthetics and inherent product usability. A 2 x 2 x 3 mixed design was used in the study, with product aesthetics (high / low) and usability (high / low) being manipulated as between-subjects variables and exposure time as a repeated-measures variable (3 levels). A sample of 60 mobile phone users was tested during a multiple-session usability test. A range of outcome variables was measured, including performance, perceived usability, perceived aesthetics, and emotion. A major finding was that the positive effect of an aesthetically appealing product on perceived usability, reported in many previous studies, began to wane with increasing exposure time. The data provided similar evidence for emotion, which also showed changes as a function of exposure time. The study has methodological implications for the future design of usability tests, notably suggesting the need for longitudinal approaches in usability research.Keywords: usability; aesthetics; longitudinal evaluation; field study; user experience Practitioner SummaryThis study indicates that product aesthetics influences perceived usability considerably in one-off usability tests but this influence wanes over time. When completing a usability test it is therefore advisable to adopt a longitudinal multiple-session approach to reduce the possibly undesirable influence of aesthetics on usability ratings.