Learning physics in any context, including undergraduate research experiences (UREs), requires learning its concepts and the relational structure between those new concepts with what students already know. We use concept maps, a knowledge elicitation method, for assessing mentees' and mentors' knowledge structures during Research Experience for Undergraduates programs. The study looked at maps from seven mentor-mentee pairs to understand how mentors and mentees use specific knowledge and strategies during the development of their concept maps. A qualitative analysis of the maps showed mentors and mentees differed in their ways of organizing and displaying their knowledge in terms of structure, scale, language, and use of conceptual and procedural knowledge. For instance, mentees used more procedural knowledge. It is perhaps due to their perception of finishing their REU projects and the fact that they may have only limited and superficial knowledge of specific topics. However, mentors' maps were smaller but more significant in using more comprehensive conceptual knowledge and connecting their maps to the broader scientific context.