The influence of ultraviolet (UV) illumination on the formation of hydroxyl radicals in dielectric barrier discharges (DBD) in mixtures ofAr/O2/H2O(79/18/3) will be presented. The UV is provided from a spectrally calibrated, commercial low pressure mercury lamp while the irradiance power of the discharge cell has been previously determined. First, the filamentary character of the DBD is shown by measurement of the relative intensity of the transitionOH(A2&Sgr;→X2&Pgr;, 0-0)with integrated emission spectroscopy as a function of the absolute water concentration at different energy densities coupled into the gas. Then, the influence of UV illumination on the hydroxyl emission is shown by time-gated and time-resolved emission spectroscopy ofOH(A2&Sgr;→X2&Pgr;).The results of the combined process is an increased total emission but reduced emission for each microdischarge. This result corresponds to an increase in the number of microdischarges which is supported by the measurement of the microdischarge current. ©1997 American Institute of Physics.