In the present study, we investigated whether online learning behaviours (navigation and writing activities) mediated the relation between learner characteristics (prior knowledge, vocabulary knowledge, working memory, and motivation) and declarative knowledge. Specifically, we investigated whether the quality of participants' written assignments could further explain this relation. For this purpose, 62 fifth-grade children participated in a WebQuest hypermedia assignment on the subject of the heart. Results showed that online learning behaviours did not mediate the relations between learner characteristics and declarative knowledge when the assignment quality was not included in the model. Adding assignment quality, however, revealed two serial mediation models. Prior knowledge predicted declarative knowledge via the writing activity "copying behaviour" and assignment quality, and vocabulary knowledge predicted declarative knowledge via the navigation activity "time spent on assignment pages" and assignment quality. These results show the importance of taking into account assignment quality when investigating knowledge acquisition in hypermedia environments.