We have studied the long-term stability of a set of CuIn1-xGaxSe2 solar cells with different Ga-contents x = (0, 0.3, 0.4, 0.5, 0.6, 1) during 800 h of accelerated ageing in both dry and damp heat (DaH) conditions. Device performance has been monitored with quantum efficiency and current-voltage measurements at varying temperatures (J(V)(T)). All samples but that with x = 1 are stable in dry heat conditions. After 800 h of DaH exposure, the samples with Ga contents close to 0.4 show the smallest efficiency degradation of approximately 25% relative to their initial performance. The samples with x = 0 and x = 1 have degraded by approximately 90 and 65%, respectively. Analysis of the J(V)(T) measurements reveal a blocking behavior and enhancement of a second diode mechanism with high ideality factor, together explaining the loss in the solar cell parameters. We suggest that the second diode be assigned to tunneling-enhanced recombination at the CdS/CIGS interface. (C) 2003 Elsevier Science B.V. All rights reserved.