Crystallographic phase separation and optical properties of ZnO1-xSx (x=0.1-0.3) alloy thin films grown by pulsed laser deposition (PLD) on c-plane sapphire substrate have been investigated with and without using ZnO buffer layer between the film and substrate. ZnO buffer layer assists in lattice matching of the alloy thin film and influences the phase separation and incorporation of S in this system. ZnO0.7S0.3 thin film separated into four different phases when directly grown On c-plane sapphire compared to two when grown using 8 nm thick ZnO buffer layer. ZnO0.88S0.12 thin film showed a single phase with tilt between crystallites but forms epitaxial film without any tilt on ZnO buffer layer. Incorporation of S is close to the targeted value for the films grown with the buffer layer. With the help of buffer layer we have been able to grow epitaxial film up to 17 at S. Band gap for each alloy phase has been determined by low loss electron energy loss spectroscopy and a band bowing parameter of 5 eV is obtained. (C) 2014 Elsevier B.V. All rights reserved.