The effect of thermal annealing and irradiation in an intense white synchrotron x‐ray beam on the x‐ray reflectance of tungsten/carbon and tungsten/silicon multilayers is reported. Thermal annealing at 400 °C for two hours produces larger effects than irradiation of cooled multilayers in the white beam of a 20‐pole hard x‐ray wiggler with 0.94‐T peak field on the storage ring DORIS operating at 5.42 GeV and electron currents of 20–36 mA for 40 h. Thermal annealing caused the period and first order reflectance of a W/Si sample to decrease, in contrast to a W/C sample whose period and reflectance increased on annealing. Of five actively cooled samples irradiated, one W/C sample showed significant change in reflectance. Preannealing of this multilayer stabilized it to radiation‐induced changes. Irradiation effects also depend on multilayer period and constituent materials. Implications of these results for models describing multilayer reflectance and for multilayer applications in the new generation of synchrotron radiation sources are discussed.