We have investigated exciton dynamics and spin relaxation in a GaAs/AlAs multiple quantum well and in tensile-strained GaAs1−xPx/Al0.35Ga0.65As quantum wells. The studies have been done by picosecond time-resolved photoluminescence spectroscopy. The strain introduced by the presence of phosphorous in the GaAs1−xPx/Al0.35Ga0.65As quantum wells modifies the subband dispersions, permitting access to new structures that include the possibility of the light-hole exciton's being the first excited state of the system. Our studies have focused on the influence of the different subband dispersions on exciton dynamics and spin relaxation. We have found that the exciton formation and the spin relaxation times decrease for the quasidegenerate heavy-hole–light-hole excitonic ground state and that the recombination time depends essentially on the character of the excitonic ground state.