Cu2ZnSnS4 (CZTS) thin films were deposited from a quaternary ceramic compound target using radio-frequency magnetron sputtering in Ar and Ar–10% H2 to investigate the effects of H2 addition to Ar on the film structure and optical and electrical properties. The sputter-deposition system was equipped with a cylindrical reflector wall between the target and substrate to enhance the probability of reimpingement of Zn and S atoms re-evaporated from the growing thin-film surface to the film surface. The thin films deposited in both Ar and Ar–10% H2 were identified as kesterite Cu2ZnSnS4 with 〈112〉 preferred orientation by X-ray diffraction measurements and Raman spectroscopy. The crystallite size evaluated from the half width at full-maximum of the 〈112〉 diffraction peak increased from 10 to 12 to 24 nm with the H2 addition to Ar at substrate temperatures &lt
200 °C. The lattice spacing of (112) also increased by 2–3 nm with the H2 addition to Ar. Cross-sectional scanning electron microscopy images of the thin films revealed that the thin-film structure changed from a fine to coarse columnar structure with the H2 addition to Ar. X-ray photoelectron spectroscopy analysis indicated that the O content decreased, whereas the Cu content increased with the H2 addition. The band gap range of the CZTS thin films deposited in Ar–10% H2 was 1.4–1.5 eV, which was narrower than that of the films deposited using Ar. The Hall mobility and carrier density were not greatly affected by the H2 addition to Ar. Although other factors than O content, such as cation ratio, cation/anion ratio, and film crystallinity affect band gap and other film properties, from the fact that the H2 addition affected the lattice spacing and the band gap clearly, it was concluded that suppression of the incorporation of oxygen into CZTS thin films by the H2 addition to Ar was the foremost factor for the decrease in the lattice spacing and the narrowing of the band gap.