Measuring the spectral response of photoluminescence (SRPL) in solar cells has recently attracted attention as it can be used as a contactless relative measure of external quantum efficiency (EQE) prior to full device fabrication. However, this technique requires that the monitored PL spectrum originates mainly from a region in the solar cell with uniformly distributed majority carriers. For a stack of thin films with a similar material composition, the slightly different emission spectrum from each layer may lead to the superposition of several luminescence peaks. This letter presents the measurement of the SRPL from GaAsP tandem solar cells and outlines a method for separating the individual layer contributions. Good agreement between measured SRPL and EQE at short wavelengths has been achieved, and the deviations at longer wavelengths have been analyzed. This study also reveals unexpected bandgap narrowing resulting from a variable material composition within the active region.