ZnSSeO quaternary alloys were grown on GaP substrates by molecular beam epitaxy and the group VI compositions were controlled. Reductions of the supply amounts of Se and S both increase the O concentration. S concentration were decreased by the decrease of S supply and increased by the decrease of Se supply. ZnSSeO alloys lattice-matched to GaP were obtained with high O and S concentrations. Dominant peaks were observed at the near band edge in the photoluminescence spectra. The band gap energies were shifted according to the group VI compositions. . For the practical application, quaternary alloy such as ZnSSeO is expected to be more useful since band gap energy can be tuned keeping the lattice constant fixed. In the previous study, we have grown ZnSe-based ZnSSeO alloys (S concentration less than 10 %) on GaAs and investigated the O 2 flow rate dependence of O concentration [3]. The band gap bowing was observed as that seen in ZnSeO. However, we have found that not only the O concentration but also the S concentration increased by increasing O 2 flow rate. Moreover, structural phase separation was observed when the O concentration was higher than 3-4 %.The most important advantage of ZnSSeO alloy is that the lattice constant can be matched to those of GaP and Si substrates. The wide range of controllable band gap energy on such substrates is attractive for the monolithic integration of opto-electronic devices. In this study, we investigated the controllability of group VI compositions in ZnSSeO alloys on GaP substrates with high S concentration.