A new technique for making viscosity measurements on gas mixtures is introduced. The composition and flowrate of a mixture flouring through a capillary tube are perturbed by adding a small stream of perturbation gas. This is usually a pure, individual component of the mixture. The pressure at the inlet of the capillary tube rises when the perturbation gas is added and this pressure increase is proportional to the flowrate change. Because there is empty volume between the point where the perturbation gas is added and the capillary tube, it is some time later that the pressure changes again when the composition of the gas flowing through the tube changes. This second pressure change is proportional to the change in viscosity. The ratio of these two steps of pressure is proportional to d ln mu/dX(i) where mu is the viscosity and Xi is the mole fraction of component i. An apparatus has been developed which is capable of making measurements with suitable precision and some preliminary data for the nitrogen-argon system at 1.2 bar and 24 degrees C are given.