In situ, line-of-sight measurements of water vapor partial pressure and temperature were performed in a gas channel on the cathode side of an operating PEM fuel cell. Tunable diode laser absorption spectroscopy was employed for these measurements for which water transitions sensitive to temperature and partial pressure were utilized. The previously demonstrated methodology for water partial pressure measurements was extended to include temperature by including additional features of the water spectra in the data analysis. The combined technique was demonstrated in a PEM fuel cell operating under both steady state and time-varying load conditions. For steady state operation, the water partial pressure increases with increasing current density on the cathode side due to production of water by electrochemical reaction. Temperature in the gas phase remains relatively constant since the fuel cell housing temperature is controlled externally. For unsteady operation of the fuel cell through a time varying current profile, it is found that the water partial pressure responds to the load changes rapidly and follows the current profile. The gas temperature varies in response to the dynamic loading and departures from steady state conditions become more apparent at higher fuel cell operating temperatures. (c) 2005 Elsevier B.V. All rights reserved.