In this paper we present a new method for determining adiabatic film effectiveness in film-cooling experiments with non-uniform inlet temperature distributions, in particular the situation of an inlet thermal boundary layer. This might arise in a quasi-steady experiment due to loss of heat from the mainstream flow to the inlet contraction walls, for example. In this situation the thermal boundary layer would be time varying. Adiabatic film effectiveness is generally normalised by the difference between mainstream and coolant gas temperatures. Most importantly these temperatures are generally assumed to be spatially—and, possibly temporally—uniform at the system inlet. In experiments with non-uniform inlet temperature, the relevant hot-gas temperature for a particular point of interest on a surface is not easily determined, being a complex function of both the inlet temperature profile and the flow-field between the inlet and the point of interest. In this situation, adiabatic film effectiveness cannot be uniquely defined using conventional processing techniques. We solve this problem by introducing the concept of equivalent mainstream effectiveness, a non-dimensional temperature for the mainstream that can be used to represent the thermal boundary layer profile at the inlet plane, or the effective temperature of the mainstream gas—which we refer to as the equivalent mainstream temperature—entrained into the mixing layer affecting the wall temperature at a particular point of interest.