Abstract: A method for determining the relative humidity of gaseous materials, preferably for determining the relative humidity of air in rooms, such as greenhouses, whereby, using psychrometric moisture measurement, the temperature of the stream of gas, the relative humidity of which is to be measured, is determined by means of temperature sensors. A gas stream acting upon a moistenable wet temperature sensor is cooled up to a cooling limit (wet-bulb point) and the wet bulb point temperature is determined. The relative humidity is ascertained using the measured temperature of the gas and the wet-bulb temperature.