We report the structural characterization of CuInS2 thin films based on Cu–In metal inks. CuInS2 films from the precursor films with different Cu/In ratios were sulfurized and investigated by SEM, XRD, Raman, and XPS. Morphological and compositional changes before and after etching of CuS impurity phase were also compared. By ex situ characterization of the CuInS2 films sulfurized at different temperatures, an established mechanism is proposed to explain the sulfurization process. The sulfurization is a diffusion‐limited process, in which the diffusion rate differences of Cu and In control the intermediate phases. Moreover, the Cu/In ratio and sulfurization temperature determine the morphological and structural qualities of the CuInS2 films.