["Human Resource Development Quarterly, EarlyView. ", "\nABSTRACT\nThis paper complements retrospective approaches to researching informal learning in the workplace with experience sampling. Since (conscious) informal learning is becoming increasingly important for successfully keeping pace with rapid changes in working environments, a clear understanding of the construct and its precise measurement are essential. This is opposed by the error‐proneness of retrospective measurements. The aim of this study was therefore to better understand how employees learn informally by situationally measuring their use of informal learning activities (e.g., searching the internet). Three hundred sixty four full‐time employees with desk jobs in various professions completed short online questionnaires twice a day for 8 days about their current work task and the informal learning activities used. A Multilevel CFA with good model fit revealed the same three factors at the situation and person level: learning from oneself, learning from others, and learning from media. Thinking/reflection, professional exchange/co‐working with colleagues, and sharing knowledge were most frequently reported. Learning activities with media were used comparatively little. These findings underscore the metacognitive role of reflection in informal learning, challenge previous models of informal learning that underrepresent situational antecedents and learning from media, and emphasize the importance of including multilevel data in research. Our research provides human resource development professionals and supervisors with a foundation for promoting informal learning and the society with basic knowledge for familiarizing students with the need for informal learning. This ability to adapt quickly to changing knowledge requirements is crucial to the competitiveness of many industries.\n"]