Humidity or a gas concentration or a solvent concentration in at least one gas is detected by a field effect transistor-based gas sensor whose sensor signal is generated by the change in the work function on a sensitive film. Detection is to be provided in a simple, effective and inexpensive manner. An additional change in potential is impressed at a gate of the field effect transistor and a variable of the resulting change in the sensor signal relative to the additional change in potential is evaluated. For example, each variable, which is e.g. a ratio, can be assigned a relative humidity, a gas concentration, or a solvent concentration. Sensitive films having at least one polymer are particularly advantageous.