An area of major importance relating to prototype developments in the photovoltaic field is accurate measurement of photovoltaic devices and modules. In this paper are described some measurement irregularities that can affect performance testing of CdS/CdTe devices and modules, and therefore can affect life testing and technical developmental decisions. These irregularities are dependent on the material being tested and on the measurement technique. They especially relate to devices filled with deep-level traps (large diode ideality factors) that can result in an inability to instantaneously equilibrate to an existing or changing bias condition.