Using CdS compound and elemental Se, li e have grown CdSc/ZnSe quantum structures with improved optical and structural properties exploiting an exchange reaction which leads to the substitution of sulfur by selenium. Typical S contamination is below 2%. A possibly enhanced surface diffusion of adatoms caused by the high CdS oven temperature and a surfactant-like effect of the S-Se exchange lead to a suppression of Cd segregation in the case of migration enhanced epitaxy with long Se exposure times. The new growth method leads to CdSe quantum wells with outstanding optical quality. Their properties are compared to CdSe island structures obtained in a non-migration-enhanced growth mode.