Abstract: The quality of electrochemical measuring sensors with at least one measuring electrode, for example pH-sensors is checked in that the frequency response of the sensor impedance is measured over a frequency range (f1, f2). The frequency response or the value of an equivalent circuit diagram of the measuring circuit (1) computed from the frequency response are compared to reference values. Deviations from the reference values indicate an impairment or damage of the measuring electrode (2). Simultaneously proceeding from the membrane impedance the temperature (T) of the measuring electrode (2) and thus of the measuring fluid (F) is determined.