Recent research has presented a conceptualization, metric, and instrument based on Microsoft Usability Guidelines (MUG; see Agarwal and Venkatesh 2002). In this paper, we use MUG to further our understanding of Web and wireless site use. We conducted two empirical studies among over 1,000 participants. In study 1, conducted in both the United States and Finland, we establish the generalizability of the MUG conceptualization, metric, and associated instrument from the United States to Finland. In study 2, which involved longitudinal data collection in Finland, we delved into an examination of differences in factors important in determining Web versus wireless site usability. Also, in study 2, based on a follow-up survey about site use conducted 3 months after the initial survey, we found support for a model of site use that employs the MUG categories and subcategories as predictors. The MUG-based model outperformed the widely employed technology acceptance model both in terms of richness and variance explained (about 70 percent compared to 50 percent).