We report measured temperatures inside the single polymer fuel cell, and thermal conductivities and heat transfer coefficients calculated from these. Temperatures were measured next to the membrane on its two sides, and in the gas channels. Higher temperatures (5 degreesC or more at 1 A/cm(2)) were found at the membrane electrode surface than in the gas channels. The thermal conductivity of the membrane (lambda(m)) was small, as expected from the properties of water and polymer, while the heat transfer coefficient of the electrode surfaces (lambda(s)) was smaller, 1000 +/- 300 W/m(2) K for a layer thickness of 10 mum. The real coefficient is smaller, since the measured temperatures are systematically smaller than the real ones. The electrode surface heat transfer coefficient is not previously reported. The average value for the catalyst surface plus gas diffusion layer was 0.2 W/m K. (C) 2003 Elsevier Ltd. All rights reserved.