Perturbation viscometry measures the logarithmic viscosity gradient of the viscosity–composition curve for gas mixtures. The flow and composition of a gas mixture flowing through a capillary are perturbed by adding a small flow of gas, normally one of the pure components of the gas mixture. Two pressure changes are seen at the capillary, the first due to the change in flow rate and the second, a short time later, due to the change in viscosity. The logarithmic viscosity gradient can be calculated from the ratio of these two pressure changes. Integration of logarithmic viscosity gradients measured over the full composition range gives the mixture viscosity relative to the viscosity of one of the pure components of the gas mixture. This method is attractive because, for measurements of equal precision, integration of the gradients is potentially an order of magnitude more precise than conventional methods that measure viscosities directly. It can also work at high and low temperatures and perhaps high pressures. The potential of this technique was previously demonstrated with a rudimentary apparatus, but its advantages and potential flaws need to be more closely examined. Here a fully thermostatted apparatus is detailed, including all modifications incorporated from experience operating the original apparatus. Experimental data for the helium–nitrogen mixture at 360°C, the practical upper operating limit of the apparatus, are presented. Integral consistency checks performed on the measured data show that high-quality data produced compare well (less than 1% deviation) with the best literature data available.