We report molecular beam epitaxial study of wide band gap (≳2.9 eV at room temperature) MgZnSSe on (001) oriented GaAs using ZnS, Mg, Zn, and Se sources. Although the growth is under group II rich condition, the compositions of S and Mg in the MgyZn1−ySxSe1−x are linear functions of flux ratios, PZnS/PSe and PMg/PZnS, up to 35%, respectively. Mirrorlike surface and low defect density (5×104 cm−2) MgZnSSe with band gap close to 3.1 eV can be achieved. Composition modulation, tweedlike contrasts and strain contrasts in the MgZnSSe are observed from transmission electron microscope analysis. For the first time, a miscibility gap at high S and Mg compositions is reported. Nitrogen‐free radicals are used as the p‐type dopant for the doping study. For the MgZnSSe with room‐temperature band gap energy higher than 2.9 eV, the net acceptor concentration decreases as the band gap energy increases. © 1995 American Institute of Physics.