How users interact with an intelligent system is determined by their subjective mental model of the system’s inner working. In this paper, we present a novel method based on card sorting to identify such mental models of recommender systems quantitatively. Using this method, we conducted an online study ($$N \!=\!170$$N=170). Applying hierarchical clustering to the results revealed distinct user groups and their respective mental models. Independent of the recommender system used, some participants held a strict procedural-based, others a concept-based mental model. Additionally, mental models can be characterized as either technical or humanized. While procedural-based mental models were positively related to transparency perception, humanized models might influence the perception of system trust. Based on these findings, we derive three implications for the consideration of user-specific mental models in the design of transparent intelligent systems.