In this paper we report measurements of the dielectric constant of 1,1,1,2-tetrafluoroethane, HFC-134a, an environmentally acceptable refrigerant, under consideration as an alternative replacement of the chlorofluorocarbons, CFCs. The dipole moment in the gaseous phase was found to be (1.91 +/- 0.19) D, and in the liquid phase (3.54 +/- 0.01) D. We present values of the first three dielectric virial coefficients in the gaseous phase.