The influence of UV illumination on the behavior of silent electrical discharge streamers (microdischarges) in dry and humid gases (nitrogen, oxygen, air, and Ar/O2mixtures) is presented. A commercial low-pressure mercury lamp, which was spectrally calibrated, provides UV light. The UV irradiance power coupled into the discharge cell is also calculated. The transferred charge per mean microdischarge (in a voltage half-cycle) was measured for a pin electrode assembly, configured to deliver either M−G+ transitions (electrons going from metal towards dielectric) or M+G− transitions (electrons going from dielectric to metal). The total transferred charge is recorded by monitoring a charge versus voltage plot (Lissajous figure). When illumination is restricted to the gas alone, both the total transferred charge and the individual-microdischarge transferred charge remain constant for either transition (M−G+ or M+G−). However, when the dielectric is illuminated, a marked decrease in the individual-microdischarge transferred charge is observed for M+G− transitions in some gases. For this case, a greater number of microdischarges must occur in a half-cycle to keep the total transferred charge constant, thereby creating more homogeneous discharge conditions. ©1997 American Institute of Physics.