A device for monitoring the concentration of an air-vapor mixture in the fixing station of a non-mechanical printing or copying device has an ultrasonic transducer for generating and receiving an ultrasonic signal in the fixing station, the transit time of the ultrasonic signal between transmission and reception indicating the air-vapor mixture concentration in the station. The transit times are subdivided into at least three ranges of which the mean range or mean ranges identify a desired concentration. An evaluator circuit post-connected to the ultrasonic transducer forms signals corresponding to the ranges, the signals being entered in one of at least three status memories in accordance with their significance. The status memory which is filled first emits an output signal for triggering a corresponding reaction such as increasing the vapor density in the station, and cancels the contents of all of the status memories. False reactions as a result of brief and topically limited fluctuations from a mean concentration are thus largely avoided.