CdTe and CIGS type modules were tested for potential-‐induced degradation with positive and negative 1,000 V bias applied to the active cell circuit in an 85°C, 85% relative humidity environmental chamber. Both CdTe module types tested exhibited degradation under negative bias. I-‐V curve data indicated the first module type was affected sequentially by shunting followed by a recovery and then by series resistance losses; the second was affected by recombination losses. The first type showed transparent conductive oxide delamination from the glass after about 750 h of stress testing in the environmental chamber and exhibited power degradation within five weeks in field tests with -‐1,000 V system voltage. Performance of CIGS modules differed depending on the technology generation. Under negative bias, the older module design showed an initial 12% (relative) improvement, possibly because of the influx of sodium ions that has been reported to benefit the electrical properties, followed by severe degradation with continued stress testing. The newer design CIGS module exhibited the best stability of the four thin-‐film module types tested with a total loss of 9.5 % (relative) power drop after 3,100 h of test with negative voltage bias, but not clearly by system voltage stress effects considering similar behavior by a sister module in-‐chamber in open-‐circuit condition. Relative rates of current leakage-‐to-‐ground between chamber tests and modules placed outdoors under system voltage stress are compared to extrapolate anticipated coulombs transferred for given extents of degradation of the module power. This analysis correctly placed which module type failed in the field first, but overestimated the time to failure. The performance of modules at 85°C with dark current Imp applied through the cell circuit are discussed with respect to stand-‐alone fielded modules biased to near their maximum power point with load resistors.