Isothermal measurements of the partial vapour pressures have been carried out on {water + dimethylformamide (DMF)} at temperatures between T = 305.15 K and T = 323.15 K using an apparatus composed of a headspace sampler and a normal gas chromatograph. These data were simultaneously evaluated in one step with respect to the (composition dependent, integral) Flory-Huggins interaction parameter g by means of a new method which minimizes the Gibbs energy of mixing and does not require chemical potentials. The expression for g given by Koningsveld and Kleintjens, originally designed for polymer solutions, describes the present results best.