Self-categorization theory (SCT) argues that self-categorization is inherently variable and tied to changes in the intergroup context (Oakes, Haslam, & Turner, 1994; Turner, Oakes, Haslam, & McGarty, 1994). Two studies were conducted to investigate SCT's claim that self-categorization is context dependent (Study 1) and to identify factors that may affect the likelihood of context effects on self-categorization (Study 2). In the first study, Scottish students self-categorized themselves at the national and European levels of abstraction in four between-participant conditions: after stereotyping the English, after stereotyping the Germans, after stereotyping the Australians and in isolation. Context effects on self-categorization were found but only at the European level of abstraction. European identity decreased significantly with the inclusion of the English and Germans in the frame of reference, but remained stable with the Australians in the comparative context. These results suggest context effects on self-categorization are not necessarily straightforward and may depend upon important factors like category accessibility, category fragility and the perceived psychological relations between categories. A questionnaire was distributed to a matched sample of Scottish students (Study 2) to investigate the importance of these factors. The findings provided empirical weight to our contention that category accessibility, category fragility and perceived psychological category relations are important to the self-categorization process.