Transport and far-infrared transmission measurements are reported on Si-doped GaAs epitaxial films (n≥ the Mott density n 0 ). The spectra display a 1s→2p resonance and a Drude response that persists to low temperatures. An analysis of the temperature dependence of the spectra gives the activation energy for thermal excitation of carriers from the metallic impurity band to the conduction band. The effective mass of the impurity band m; is obtained from optical sum-rule considerations. m i appears to diverge at the metal-insulator transition. The large m i implies a correlation-induced narrowing of the impurity band. The activation energy E a a and effective mass m i obtained from optical measurements are consistent with the results obtained from transport measurements