A newly designed apparatus for accurate simultaneous viscosity and density measurements oil gases was assembled. The measuring system consists of a vibrating-wire viscometer and a single-sinker densimeter. Density values measured directly are a prerequisite for accurate viscosity measurements. For the relative measurements with the vibrating-wire viscometer, the wire radius was determined by means of a calibration with a theoretically based value for the viscosity of helium in the limit of zero density. In general, the viscosity measurements are characterized by a relative uncertainty of +/- (0.25 to 0.3)%, whereas the relative uncertainty in the density is estimated to be less than +/- 0.1%, except for the low-density range. The working range of the instrument covers temperatures from (253 to 473) K at pressures tip to 30 MPa. Some isothermal series of measurements on helium (293 K) and nitrogen [(293 and 423) K] were carried out to check the performance of the apparatus and to provide accurate eta rho pT data. On the basis of comparisons with values calculated from the viscosity surface correlations and equations of state for helium and nitrogen as well as with selected experimental data from the literature, it is concluded that the new viscosity values are appropriate to improve the viscosity surface correlations available in the literature.