The viscosity of nitrogen has been measured at temperatures in the range 120–320 K using a capillary flow technique. These results, together with previous data, are used to define a new standard for low-temperature viscosity determinations. In addition, the viscosities of the binary gas mixtures He + Ar, Ar + Kr and CH₄+ CF₄ have been measured in this temperature range using the same apparatus. Agreement to within 1% is found with the mixture data of previous workers at room temperature, and there is good agreement for the pure component viscosities over the entire temperature range. The mixture viscosities are analysed in terms of the so-called interaction viscosities η₁₂, which are related to unlike pair potential energy functions. The interaction viscosities for He + Ar are combined with previous high-temperature results and are used to obtain a potential energy function in the repulsive region.