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 O-2 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 O-2 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.