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Semiconducting ternary metal oxide thin films exhibit a promising application for solar energy conversion. However, the efficiency of the conversion is still limited by a band gap of a semiconductor, which determines an obtainable internal photovoltage for solar water splitting. In this report the tunability of the tin tungstate band gap by O 2 partial pressure control in the magnetron co-sputtering process is shown. A deficiency in the Sn concentration increases the optical band gap of tin tungstate thin films. The optimum band gap of 1.7 eV for tin tungstate films is achieved for a Sn to W ratio at unity, which establishes the highest photoelectrochemical activity. In particular, a maximum photocurrent density of 0.375 mA cm−2 at 1.23 V RHE and the lowest reported onset potential of −0.24 V RHE for SnWO 4 thin films without any co-catalyst are achieved. Finally, we demonstrate that a Ni protection layer on the SnWO 4 thin film enhances the photoelectrochemical stability, which is of paramount importance for application. Image 1 • Variation of oxygen partial pressure changes Sn to W ratio which tunes the band gap. • Optimum band gap (1.7 eV) results in the highest photocurrent density (0.375 mA cm−2). • The lowest onset potential of - 0.24 V RHE is achieved for tin tungstate film. • Photoelectrochemical stability of the films is improved by NiO x protection layer. • Initial open circuit potential of 0.107 V is measured upon AM 1.5 illumination. [ABSTRACT FROM AUTHOR]