The frictional stresses of poly(dimethylsiloxane) elastomers of various molecular weights were measured against a supported monolayer of hexadecylsiloxane and a thin film of polystyrene as a function of sliding velocity and temperature. On both surfaces, friction decreases with molecular weight, but increases with sliding velocity, reaches a maximum, and thereafter it decreases or displays a plateau. While the velocity corresponding to the maximum shear stress is nearly independent of the molecular weight of the polymer, it differs between the two substrates. These results are consistent with the models proposed earlier by Schallamach as well as by Chernyak and Leonov, according to which the detachment force per load-bearing chain increases with velocity while the number of chains supporting the total frictional load decreases with velocity and molecular weight. From the temperature-dependent studies, the activation energy of friction on both surfaces is estimated to be similar to25 kJ/mol, which is larger than the activation energy of viscous flow of silicone fluids, but compares well with the values obtained from recent studies of melt dynamics.