This article investigates the influence of damp heat treatments on the electronic properties of ZnO/CdS/Cu(In,Ga)Se-2 heterojunction solar cells. We observe losses of around 20% (50%) in the fill factor and between 5% (10%) in the open circuit voltage after 100 h (1000 h) of damp heat exposure. Temperature dependent current voltage measurements point to recombination in the space charge region as the dominant loss mechanism before and after damp heat treatment. Admittance spectroscopy on the heterostructures indicates a reduction of type inversion at the CdS/Cu(In,Ga)Se-2 interface as well as an increase of deep defect levels in the Cu(In,Ga)Se-2 film upon damp heat tests. We ascribe the losses in the fill factor to the detected changes at the interface and the losses in the open circuit voltage to the increased defect density in the absorber.