New measurements of the gas-phase viscosity of 1,1,1,2-tetrafluoroethane (R134a) were carried out within an international round-robin project of IUPAC, which was initiated because of the large discrepancies between the results reported by various authors for the transport properties of R134a. Six isochoric series of high-precision measurements were performed by means of a quartz oscillating-disk viscometer between 297 and 438 K and at densities from 12 to 90 mol m(-3). Isothermal values recalculated from the experimental data were evaluated with a first-order expansion for the viscosity, in terms of density, to deduce zero-density and initial-density viscosity coefficients. The accuracy of the zero-density values is estimated to be +/- 0.2%. A representation based on the new results and on reliable data sets from literature is proposed for the viscosity in the limit of zero density to replace a correlation recommended by Krauss et al. Reduced second viscosity virial coefficients deduced from the experiments are in close agreement with predictions of the Rainwater-Friend theory.