High-quality ZnS epitaxial layers were grown on (100)-oriented GaAs substrates by a low-pressure metalorganic chemical vapor deposition (MOCVD) method. Dependence of VI/II flow ratio on the 4.2K photoluminescence (PL) properties of undoped ZnS layers was particularly investigated. The PL spectrum of the excellent sample grown under optimum conditions was dominated by the radiative recombination of free and donor-bound excitons. The I_1 line appears at 332nm and becomes intense in the sample grown at relatively high VI/II ratio. We tentatively suggest that the I_1 line which originates from a deep-acceptor bound-exciton transition may be related to Zn vacancies.