The equilibrium surface (x, p, T, V-m) for water+ iso-butane was measured and additional equilibrium data for water+ n-butane are reported. Mole fractions of water for water + iso-butane from 0.5 to 0.98, pressures up to 306 MPa and temperatures up to 695 K were studied. Critical curves, represented as the envelope of the isopleths starting at the critical point of water (647 K), went through a temperature minimum and then increased at higher pressures and higher temperatures, giving evidence for (gas + gas) equilibria of the third type. The results are compared with the phase equilibrium of previously measured (water + n-alkane) mixtures. The excess molar volumes, V-E, and excess molar Gibbs energies, G(E), for water + n-butane on the phase equilibrium surfaces have been calculated for 600 and 620 K from 40 to 200 MPa. All the V-E and G(E) values were found to be positive. (C) 2003 Elsevier B.V. All rights reserved.