This article addresses the development of a transient sensible performance model for a multipass cross-flow heat exchanger where the fluids are unmixed in a pass and are mixed between the passes. Variations of the inlet fluid temperatures and mass flow rates as a function of time are considered. The article details the pertinent governing equations, and presents the results for various cases. To validate the transient performance model, the results obtained from the transient performance model for large times (where a steady solution is expected) are compared with predictions from a steady performance model.