In a device for monitoring a radio-frequency transmit device of a magnetic resonance system, having a transmitter antenna system that has a number of transmit channels, a reference scattering parameter matrix of the transmitter antenna system is determined in the unloaded state with no subject present, and a subject-specific scattering parameter matrix of the transmitter antenna system is determined in a state loaded with the subject. time-dependent transmitter amplitude vectors are determined that represent the radio-frequency voltage amplitudes on the individual transmit channels. From 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, from which monitoring values are formed that, when reached, cause operation of the radio-frequency transmit device to be limited during acquisition of magnetic resonance data.
