This paper presents a detailed study of Zn1-xMgxSySe1-y. epitaxial films on GaAs (100) substrates using photoassisted growth (250 Torr, 350 degreesC, UV = 36 mW/cm(2)) with dimethylzinc (DMZn), bismethyl-cyclopentadienyl-magnesium ((MeCp)(2)Mg), 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)(2)Mg 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)(2)Mg flows.