MgxZn1-xS mixed-alloy films have been grown on (100)-oriented GaAs substrates by low-pressure metalorganic chemical vapor deposition. Optical and structural properties of the films have been investigated by means of time-integrated and time-resolved photoluminescence (PL) spectroscopy. Up to about x(Mg) = 0.5, MgxZn1-xS films were crystallized in zincblende phase. With increasing Mg composition ratio above I,, = 0.5, both zincblende and wurtzite structures coexisted. It was found from time-resolved PL measurement that the main emission was due to the recombination of localized excitons, which was caused by alloy fluctuation. It was also found that the degree of localization increased with the Mg composition ratio.