•The variation of electronic structure of CZTSSe films with S/(S + Se) ratio x is studied.•The monotonous rise of the conduction band minimum with x is clarified.•The band alignment at the CdS/CZTSSe interface is clarified by in-situ PES/IPES.•The change of the conduction band offset from positive to negative as an increase of x is observed.•The consistency between the band alignment and the cell performance is confirmed.The dependences of electronic structure of CZTSxSe1 − x (CZTSSe) layers synthesized by sulfurization and/or selenization of the vacuum-deposited metal precursors on the anion mixing ratio x = S/(S + Se) have been studied by in-situ ultraviolet and X-ray photoemission spectroscopies (UPS, XPS) and inverse photoemission spectroscopy (IPES). The band alignment at interfaces between the CdS buffer by the sequential evaporation and the CZTSSe (x = 0.28 and 1.0) has also been investigated by the in-situ measurements of these spectroscopies. The UPS/IPES results of the CZTSSe surfaces have revealed linear expansion of band gap energy Eg with an increase of x: Eg(CZTSe; x = 0) = 0.9–1.0 eV and Eg(CZTS; x = 1) = 1.5–1.6 eV. This expansion mainly originates in the rise of conduction band minimum CBM: CBM(CZTSe; x = 0) = 0.45–0.50 eV and CBM(CZTS; x = 1) = 0.95–1.05 eV. The in-situ measurements of the interface electronic structure have revealed that the CdS/CZTSSe (x = 0.28) interface has a so-called “type I” band alignment with a conduction band offset CBO about + 0.2 eV which is favorable to high cell performance. A negative CBO was distinguished for the CdS/CZTS (x = 1.0) interface, and the observed change in the band alignment with the anion mixing ratio was consistent with that of the variation in cell-performances.