Equilibrium compositions of vapor and liquid phases were experimentally determined as a function of pressure for binary mixtures of helium + m-xylene and nitrogen + m-xylene at temperatures from 465 to 585 K. Five pressures from 50 to 150 bar were investigated at each temperature. A flow apparatus was developed for the measurement to minimize the thermal decomposition of m-xylene. The data were compared with calculated results from various equations of state. Several versions of mixing rules were also examined.