Transonic flows are very sensitive to heat release due to condensation, combustion, or electrical heating. The governing unsteady small disturbance equations for such flows are formulated in this paper. In particular, a slowly time-varying regime in which the characteristic disturbance time is much larger than a characteristic flow time is considered. In the case in which the unsteady heat release is spatially homogeneous, a similarity solution for flow in a converging-diverging nozzle has been found. The response of the flow to various forms of unsteady heat input has been computed, and the role of shock waves within the nozzle is considered.