The process and the corresponding device concern the selective detection of gases and vapours with a paramagnetic and electrical dipole moment. Using a non-uniform magnetic (31, 32, 33, 33') or electric field (41, 42, 43, 43', 44, 44'), molecules are enriched in the sound velocity-sensitive region (34) of an ultrasonic resonator (1). The ultrasonic resonator consists of two ultrasonic transducers arranged opposite each other and run as a transmitter (2) and receiver (3). The molecules are enriched by altering the sound velocity to a tuning of the ultrasonic resonator (1) and thus to a signal change in the ultrasonic receiver (3). The process is especially suitable for the selective detection of oxygen.
