To gain insight into the aging mechanisms of winding insulations for power generators, the insulation system of an in-service aged winding was assessed. Accordingly, generator bars were dismantled from an 8 kV hydrogenerator that had failed an overpotential test. Since this machine had previously been steam cleaned, it was assumed that its winding insulation could have absorbed water. A loss factor of 9.6% measured at 0.2 U(N) confirmed this assumption. To investigate the consequences of this finding, voltage endurance tests were carried out: Time to breakdown values of the in-service aged insulation were obtained that were shorter by a factor 27 compared to an unaged replacement bar. Hence, the absorption of water by an in-service aged insulation can significantly deteriorate its remaining life. Apart from the groundwall insulation, oil from the bearings of this machine had partly dissolved the corona protection of the winding. Accordingly, the maximum surface resistance of the corona protection was found to be more than ten times higher compared to its original value. This problem may yield discharges in the slots of the stator which compromise reliable operation. It is therefore recommended to check windings with similar problems for this type of discharge.