Using the method of enhanced chemiluminescence in a peroxidase-luminol-p-iodophenol system, we found the formation of reactive oxygen species (in equivalent of hydrogen peroxide concentration) in 1 mM phosphate buffer under the exposure to high peak-power pulsed electromagnetic radiation of extremely high frequencies (37 GHz, peak power 20 kW, pulse width 400 ns, repetition rate 500 Hz). The results obtained show that the formation of hydrogen peroxide in aqueous solutions under the action of electromagnetic radiation is the result of the summary influence of heat and thermoacoustic waves excited in the solutions.