The composition of vapour and liquid mixtures of nitrogen and oxygen in equilibrium at 1$\cdot $3158 atm have been measured using a dynamic method with continuous recirculation of the vapour. The vapour circuit included a quartz micro-balance which was used to obtain a non-destructive analysis of the vapour at any time during the progress of an experiment, and at equilibrium a sample of the liquid was vaporized for analysis by the same balance. The equilibrium vessel was contained in a cryostat maintained at a temperature constant to within 0$\cdot $02 degrees C and measured by means of an oxygen vapour-pressure thermometer. All the measurements, i.e. temperature, pressure and composition, were obtained in terms of the heights of columns of mercury and these heights were measured on a cathetometer to $\pm $0$\cdot $01 cm. The overall accuracy of a measurement expressed as the concentration of a component was, therefore, at least as good as $\pm $0$\cdot $1%. Tests have been applied and show that the data obtained are probably thermodynamically consistent. A table is presented giving the liquid composition for rounded values of vapour composition and the temperature at which these are in equilibrium, together with the relative volatility and its logarithm. These functions are also presented in the form of graphs.