A fuel cell system is provided, including an HFR measurement device in electrical communication with a fuel cell stack. The HFR measurement is used online to measure an HFR of the fuel cell stack suitable for calculation of a d(HFR)/d(RH) ratio. A humidity regulator is provided in fluid communication with the fuel cell stack. A controller periodically changes stack operating conditions to perturb an RH of the fuel cell stack, process the HFR response, and compute the d(HFR)/d(RH) ratio. A method for online identification and control of the fuel cell stack humidification is also provided. The d(HFR)/d(RH) ratio is an auxiliary measurement of membrane hydration which is used as a feedback for hydration control.