Perturbation viscometry measures the pressure changes in a gas mixture flowing through a capillary tube when the flow is perturbed by adding a small flow of one of the pure components. The pressure upstream of the capillary rises because of the increase in flow and then rises or falls because of the change in viscosity. These pressure changes are measured accurately and their ratio gives the rate of change of viscosity with composition. The relative viscosities of the pure components of the gas mixtures are then calculated by integration of the gradients measured across the composition range. Measurements of the viscosity-composition relationships for binary gas mixtures of argon, nitrogen and helium at 98.5degreesC have been made together with some measurements on ternary mixtures. Perturbation viscometry gives more information than needed to just calculate the viscosity ratios. The extra information can be used in a series of checks to confirm the accuracy and consistency of the data produced. Consistency tests originally developed for binary gas mixtures have been applied to the measurements. Four more tests specific to ternary mixtures have been devised. The most rigorous of these involves integration of the gradient data between the same points over different paths. The closure error quantifies the error in each of the calculated relative viscosities. These tests show the results to be of very high quality with errors of less than 0.2%. The ability to identify and quantify the errors in this way is unique to perturbation viscometry and gives additional confidence. The regressed viscosity ratios have been used to prepare an outline viscosity-composition surface of the ternary mixture argon-nitrogen-helium at 98.5degreesC. (C) 2003 Elsevier B.V. All rights reserved.