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We examine the status and question of long-term stability of copper indium diselenide (CIS) photovoltaic (PV) module performance for numerous modules that are deployed in the array field, or on the roof of, the outdoor test facility (OTF) at NREL, acquired from two manufacturers. Performance is characterized with current-voltage (I–V) measurements obtained either at standard test conditions (STC) or under real-time monitoring conditions, taken over the course of many years. We present and scrutinize I–V characteristics for degradation modes. Analysis yields that CIS PV modules can exhibit either moderate (2% to 4% per year) to negligible or small (less than 1% per year) degradation rates, and that the predominant loss mode appears to be fill factor diminution, often associated with increases in the series resistance in some of the modules. A secondary mode of degradation observed comprises metastable changes to the open-circuit voltage. The featured modules are deployed on three separate testbeds.