MgxZn1-xS mixed films have been grown on (1 0 0)-oriented GaAs substrates by low-pressure metalorganic chemical vapor deposition. In the range of Mg composition (x) up to about 0.4, the mixed crystals were grown only in the zincblende phase. The 4.2 K photoluminescence spectrum in the crystal with x(Mg) = 0.39 was dominated by the strong ultraviolet emission around 300 nm. The growth of CdyZn1-yS/ZnS/MgxZn1-xS separate confinement heterostructures (SCHs) was performed and ultraviolet emission at about 340 nm from a Cd(0.23)Z(0.77)S well layer in SCH was observed at RT for the first time.