The influence of accelerated stress testing on the electrical properties of Cu(In,Ga)(S,Se)(2) thin-film solar cells is presented. Although encapsulated modules are stable, extended exposure of unencapsulated cells to damp heat (144 h, 264 h and 438 h) at 85degreesC and 85% humidity leads to a reduction of the fill factor and the open-circuit voltage. We further analyzed these changes by applying admittance spectroscopy, deep-level transient spectroscopy and current-voltage measurements. Damp heat stress reduces the net doping concentration of the absorber material and shifts a dominant defect state close to or at the buffer/absorber interface to higher activation energies. By comparing experimentally observed changes with the SCAPS device simulation program several possible causes for the observed changes in electrical properties are discussed: decrease of ZnO resistivity; decrease of the density of interface states; increase of shallow acceptor concentration and the introduction of deep acceptors in the Cu(In,Ga)(S,Se)(2) absorber.