A relative humidity (Φ) of the environment of a humidity sensor (1) is measured by the humidity sensor (1) twice at different temperatures (T). For both measurements, a corresponding value (V) based on a saturated vapour pressure at the subject temperature (T) is calculated. The two calculated values (V1, V2) derived are then evaluated. The evaluation result allows for a conclusion if the humidity sensor (1) may be impaired in its capabilities of measuring humidity.