Isothermal vapour-liquid equilibrium data of three binary systems containing water, dimethylsulphoxide and 1,4-dioxane and of corresponding ternary systems have been measured using the total pressure method. The results were correlated using the Wilson model. The pressure of the ternary system was also predicted at 313.15 K using the Wilson model with parameters determined from binary vapour-liquid equilibria. The predicted vapour pressures agree satisfactorily with the measured values.