A widespread call has been made to develop and support more integrated approaches to STEM education. The first law of thermodynamics serves as a guiding principle for the crosscutting concept of energy and matter. A qualitative interview study was undertaken to support integrated approaches to STEM education by exploring how chemistry, physics, and engineering students (n = 40) transfer first law concepts across disciplinary contexts. Acquired interview data were analysed through the lens of the dynamic transfer framework to reveal the underlying contextual elements students used to know with. Emergent trends across the disciplines revealed how these applied reasoning approaches and epistemologies were realised by each discipline. Productive transfer is shown to be facilitated by the coordination of different disciplinary epistemologies. Suggestions are made to practitioners on how to support students in applying different reasoning approaches when addressing first law problems. The applied methods also serve as a promising methodology for future investigation of students' transfer of crosscutting concepts.