Photoluminescence (PL) properties of ZnO films grown epitaxially on lattice-matched ScAlMgO4 substrates are investigated. PL spectra of ZnO layers measured at 5 K are dominated by neutral-acceptor bound exciton emission at 3.359 eV (I-6) with a linewidth of 0.8 meV. Free exciton emission develops as temperature is raised and eventually dominates at temperatures higher than 120 K. Temperature dependence of intensity of the free exciton emission is discussed.