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We have measured the viscosity of thin polymer films on Si substrates using three independent, yet complementary techniques:  bilayer dewetting measurements where the viscosity of the lower layer is derived from the opening velocity of dewetting holes in the more viscous upper layer, X-ray photon correlation spectroscopy (XPCS) where the viscosity of a single layer is determined from the relaxation rate of thermally induced surface roughness, and dynamic secondary ion mass spectroscopy (DSIMS) where the viscosity is derived from measurements of the tracer diffusion coefficient. The scaling relationship, η ∼ M_wPS^α, yielded α = 3.3 ± 0.3 and α = 3.2 ± 0.1 from dewetting and XPCS measurement, respectively, which was in excellent agreement with the bulk scaling of 3.4 and the prediction from reptation theory. The absolute magnitudes were consistently higher by at most a factor of 3 than the bulk values …