This paper presents a detailed study of Zn1−xMgxSySe1−y epitaxial films on GaAs (1 0 0) substrates using photoassisted growth (250 Torr, 350°C, UV=36 mW/cm2) with dimethylzinc (DMZn), bismethyl-cyclopentadienyl-magnesium ((MeCp)2Mg), diethylsulphide (DES), and dimethylselenide (DMSe) as precursors. We achieved up to 12% Mg incorporation (with 12% S incorporation) as well as reproducible growth of lattice matched Zn0.94Mg0.06S0.11Se0.89 layers. The variance in the Mg and S incorporation does not increase with increasing Mg incorporation, and the FWHM of the X-ray diffraction peak of our lattice-matched films is below 50 arcsec. Furthermore, the FWHM of the near band edge emission (NBE) as measured by PL does not increase with increasing Mg incorporation. Although the S incorporation was not completely independent of the (MeCp)2Mg flow, we believe that we can engineer both the band gap and lattice constant of the Zn1−xMgxSySe1−y films by proper adjustment of the DES and (MeCp)2Mg flows.