Chemical bath deposition (CBD) is commonly used to deposit ZnS thin films as buffer layers in thin film solar cells, but oxygen is often incorporated into the film as oxide and hydroxide to form Zn(S,O,OH). Efforts to understand the gradation of the film stoichiometry and properties are limited by film thicknesses of ∼50 nm that are smaller than the probe size of many characterization techniques. We use a continuous flow microreactor (CFμR) to investigate relationships between bath composition and film properties by transposing through-plane gradients over ∼50 nm into lateral gradients over centimeters. Zn(S,O,OH) films were deposited on glass, Cu 2 (Zn, Sn)(S,Se) 4 , and CdSe using thiourea (TU) and thioacetamide (TAA) sulfur sources. X-ray photoelectron spectroscopy (XPS) shows increasing S/(S+O) with distance for TU films and the opposite trend for TAA films, spanning a range of 0.42−0.59 on a single substrate. Films on glass comprise highly monodispersed nodules, revealing separate nucleation and growth regimes. Experimental bath sulfide concentration and pH data were incorporated into speciation models, which showed that Zn(OH) 2 governs nucleation, whereas ZnS promotes growth. The CFμR provides unique insight into CBD of Zn(S,O,OH) thin film deposition for optimal control of film morphology and stoichiometry for photovoltaic buffer layers.