A new method to determine the composition of binary gas mixtures is described. In one mode, the method is absolute and does not require calibration. The essential requirement is a detector to indicate changes of a physical property that varies with composition. Initially, the binary mixture of unknown composition hows through the detector and the detector signal is steady. If the composition of the binary mixture is changed, the physical property of the mixture will change and this will lead to a change in the detector signal. Two separate experiments are required to determine the composition of the binary mixture. In each experiment, a composition change is made by adding a small (perturbation) flow. These perturbations are usually each of the two pure components. From the ratio of the two changes in detector signal and the ratio of the two perturbation flowrates, the composition of the unknown mixture can be determined. In this investigation, the physical property was thermal conductivity and the 'unknown' mixture consisted of nitrogen and argon for which the nitrogen mole fraction was certified to be 0.497 +/- 0.005. The nitrogen mole fraction was determined as 0.494 +/- 0.003.