Zinc oxysulfide (Zn(O,S)) is widely used for photovoltaic and optoelectronic devices because its electronic properties are tunable with adjustments to the S-to-O composition ratio. Zn(O,S) thin films used in devices are typically assumed to have constant S-to-O composition ratios across their thicknesses. However, S-to-O composition ratio gradients, and thus electronic property variations along the vertical direction, can be naturally induced. Such gradients can enhance device performance. In this work, we analyzed the S-to-O composition ratios along the thickness directions of Zn(O,S) thin films deposited at a fixed O2 gas flux. Natural O enrichment was observed near the bottom of the film, attributed to the highly reactive nature of the sputtering process. By increasing O2 gas flux during sputtering, more compositionally uniform thin films were obtained. We suggest that non-uniform phase distribution in the depth direction could be considered for achieving desired composition ratios when depositing Zn(O,S) thin films using reactive sputtering.