This paper examines the impact of thinking-aloud (TA) instructions as well as of individuals' prior domain knowledge on information processing and source evaluation during Web search on a health-related topic. With regard to TA instructions, prompted instructions that entailed evaluation prompts (as used in some previous Web search studies) were compared to neutral instructions (in line with the standards defined by Ericsson & Simon, 1993) and to a silent condition. To measure participants' (N = 44) information processing and source evaluation we used a rich multi-method approach including eye-tracking methodologies, log file data, and verbal protocols. Results indicate that prompted TA instructions as compared to neutral instructions significantly increased participants' verbal reflections on information quality and on structural aspects of Web pages, given that participants possessed at least a moderate level of prior domain knowledge. In addition, prompted instructions resulted in less linear viewing sequences on the search engine results pages than the silent condition. Finally, the higher participants' prior domain knowledge the more intensely they scrutinized the search results presented by the search engine and the smaller were their average pupil sizes, which indicated lower cognitive load. The significance of the results is considered in light of methodological as well as educational implications.