Sourcing is vital for knowledge construction from online information sources, yet learners may find it difficult to engage in effective sourcing. Sourcing can be particularly challenging when lay readers encounter conflicting expert accounts of controversial topics, a situation which is increasingly common when learning online. The aim of this study was to examine learners' spontaneous sourcing as they read divergent expert accounts of a socio-scientific controversy in order to map prevalent sourcing practices and to identify specific challenges. Additionally, the study explored the role of learners' epistemic perspectives in sourcing, and examined the relations between sourcing while reading and subsequent written argumentation. Sixty-one university students thought aloud while reading four conflicting blog-posts about a socio-scientific controversy and then wrote arguments regarding the controversy. The findings revealed a wide range of sourcing practices. Some participants did not explicitly engage in sourcing while reading, whereas others formed detailed source representations, source-content links, and source-source links. Although most participants constructed source representations, these representations were infrequently acted upon. Multiplism was negatively related to sourcing and positively related to reliance on the reader as a source of knowledge. Higher levels of sourcing were related to more complex argumentation, increased claim justification, and better integration of sources in participants' arguments. The theoretical and instructional implications of these findings are explored.