We report results of electron paramagnetic resonance, photothermal deflection spectroscopy, and capacitance‐voltage measurements on amorphous hydrogenated silicon nitride (a‐SiNx:H) thin films exposed to ultraviolet (UV) illumination. It has been previously shown that exposure to UV light activates silicon dangling‐bond defects, i.e.,K0centers, ina‐SiNx:H thin films. Here, we demonstrate that the initially UV‐activatedK0center can be irreversibly annihilated at long illumination times. Because this effect seems to scale with H content of the measured films, we propose that hydrogen may be passivating theK0defects during the extended UV exposure. We also show that films subjected to long UV exposures trap charge as efficiently as those having much largerK0concentrations. A few possibilities to explain this effect are discussed. ©1995 American Institute of Physics.