The range of a previously developed capillary-flow viscometer has been extended to − 100°C, and the techniques of obtaining and determining the attainment of steady state have been improved. The viscosities of nitrogen, helium, hydrogen, and argon have been measured by an absolute method from − 100 or − 90° to 25°C and up to 150–175 atm (250 atm for N2 at 25°C). The accuracy is estimated to be 0.1%–0.2%, but is somewhat worse for Ar at − 100°C. A number of empirical and theoretical analyses of the viscosity–density dependence have been made; the evidence is inconclusive as to the presence of a logarithmic term in the density expansion. Correlations of the second and the third virial coefficients of viscosity have been made.