Cu(In, Ga)Se-2 (CIGS) layers were grown on ZnO for superstrate thin film solar cells. With analytical electron microscopy, a continuous Ga accumulation of 15-60 nm thickness was observed at the ZnO-CIGS interface. This accumulation was identified as a separate layer between ZnO and the CIGS absorber layer and coincides with an increase in the oxygen concentration. X-Ray photo-electron spectroscopy measurements at the ZnO-CIGS interface revealed a decrease in the binding energy of Ga with increasing depth away from the interface. This change in binding energy is attributed to a change in Ga from an oxidized state to the selenate state in the bulk absorber. A comparison of different substrates and Rutherford back scattering measurements on the ZnO layer strongly suggest loosely-bound oxygen in ZnO, leading to the oxidation of Ga, and hence, to the segregation of Ga,03 at the ZnO-CIGS interface.