Thin films of a-Si:H are deposited on substrates at 300 degrees C by a conventional thermal evaporation technique. The electrical conductivity of these films is modified by the addition of antimony giving n-type films. The optical properties of the films are investigated using spectrophorometric measurements of the transmittance and reflectance in the wavelength range 200-3000 nm. Both the refractive index n and the absorption coefficient alpha increase when the Sb content is increased. The absorption edge shifts to lower energies for doped films. The optical gap E-g is evaluated using three different plots for comparison, namely; (alpha hv)(1/2), (alpha/hv)(1/2) and (alpha hv)(1/3). The value of E-g decreases with doping for the three expressions. The Urbach parameter E-0 is calculated and found to increase with doping from 74 meV for the undoped film to 183 meV for concentrations of 9.4 at. % Sb.