In an air conditioner for a motor vehicle, which, for the purpose of contling the humidity, has in the ir duct suitable devices, controlled by a microcomputer. The microcomputer receives values for outside temperature, inside temperature and inside humidity, measured by sensors. From the values for outside temperature, inside temperature and inside humidity, the microcomputer determines the threshold humidity (l.sub.thresh) as the ratio of the saturated vapor pressure (P.sub.S) inside at the pane and the saturated vapor pressure (P.sub.i) of the inside air. If this difference (.DELTA.l) between threshold humidity (l.sub.thresh) and inside humidity (l.sub.i) falls below a stipulated value (x), dehumidification devices are actuated. If the stipulated value is exceeded, humidification devices are actuated.