The naphthalene sublimation technique and numerical solution of the governing equations are used to study the mechanism of heat transfer in the radial grooves of cooled disc friction pairs. It is shown that a main eddy generated by the relative motion of the friction pair creates an effective mechanism of heat transfer to the fluid flowing in the radial grooves. The effect of Coriolis forces and a thin film separating the plates on the heat transfer coefficient are investigated. Numerical and experimental Sherwood numbers for mass transfer agree to within the estimated experimental accuracy of 12%. Sample numerical results are used to describe the detailed behavior of the system.