In a method and device for monitoring a radio-frequency transmit device of a magnetic resonance tomography system, having a transmitter antenna system that has a number of transmit channels, during a magnetic resonance measurement of an examination subject, a reference scattering parameter matrix of the transmitter antenna system is determined in the unloaded state, and a subject-specific scattering parameter matrix of the transmitter antenna system is determined in a state loaded with the subject of examination. Moreover, transmitter amplitude vectors are determined in time-dependent fashion that represent the radio-frequency voltage amplitudes on the individual transmit channels. On the basis of the subject-specific scattering parameter matrix, the reference scattering parameter matrix, and the transmit amplitude vectors, radio-frequency power values absorbed at particular transmit times in the subject are determined. Based on a large number of the determined radio-frequency power values, a number of monitoring values are formed. The radio-frequency transmit device is limited in its functioning when a monitoring value reaches or exceeds a prespecified boundary monitoring value.