We present a kinetic theory of low-temperature localized-exciton energy relaxation in the multiple hopping regime under steady-state and pulsed optical excitation. The theory permits to relate the lineshape and the temporal redshift of photoluminescence spectra to the exciton kinetic parameters and band-tail density of states. A series of (Zn,Cd)Se/Zn(S,Se) quantum wells of different thickness and content has been studied using cw and time-resolved photoluminescence techniques. A 20 Å Zn0.8Cd0.2Se/ZnSe quantum well is selected for the detailed comparison between experiment and theory and for the evaluation of localized-exciton parameters.