The k.p method and the effective mass theory are applied to compute valence-band electronic structure and optical properties of ZnO1-xSx ternary alloys under biaxial strain. A significant modification of the band structure with increasing sulfur content is revealed. Features of wave-functions and matrix elements in the transverse electrical (TE) and transverse magnetic (TM) regimes for three valence subbands are studied and discussed. The results of calculations of interband transition energy and spontaneous emission spectra are in agreement with experimental data for ZnO1-xSx films grown by radio-frequency magnetron sputtering technique. (C) 2015 Elsevier B.V. All rights reserved.