An MOCVD reactor such as a rotating disc reactor (10) is equipped with a gas injector head having diffusers (129) disposed between adjacent gas inlets. The diffusers taper in the downstream direction. The injector head desirably has inlets (117) for a first gas such as a metal alkyl disposed in radial rows which terminate radially inward from the reactor wall to minimize deposition of the reactants on the reactor wall. The injector head desirably also has inlets (125) for a second gas such as ammonia arranged in a field between the rows of first gas inlets, and additionally has a center inlet (135) for the second gas coaxial with the axis of rotation.