At 60℃

the total pressure of dimethylformamide with styrene and o-xylene respectively was measured. A nonathermal associated solution model Was used to treat the pressure-composition data

and the associated constants were listed. It has shown that this model is better than the regular associated model for the systems in which the mole volumes of the components of the binary solution are quite different. The calculated results are in agreement with the experimental results of vapour-liquid equilibrium by flowing method at constant temperature.