Metalorganic vapor-phase epitaxy of ternary ZnMgSe layers and, for the first time, quaternary ZnMgCdSe superlattices is presented. Lattice-matched Zn0.14Mg0.86Se with a high magnesium content I and a good structural quality was achieved by introducing a lattice-matched ZnCdSe buffer and phosphorus stabilisation during InP deoxidation. We obtained a rocking curve half-width below 100 arcsec for a 300 nm thick layer with x = 0.86. Lattice matching on InP was achieved for ZnMgCdSe layers with magnesium concentrations from 38% to 68%, corresponding to a bandgap of 2.9 and 3.4eV, respectively. An overall optical gain of 35 cm(-1) was observed in a ZnMgCdSe superlattice structure.