Bulk (vapour–liquid equilibrium and excess enthalpy data) and surface (surface tension) properties of the highly non-ideal associated mixtures formed by methanol and propanal have been measured. The vapour–liquid equilibrium data have been obtained at 288.15, 298.15, 308.15 and 318.15 K using a Gibbs–Van Ness type apparatus. These mixtures show negative deviations from ideality, including a negative azeotrope in the methanol-rich region. The calculated values for the excess Gibbs energy are very large and negative, and their absolute value decreases sharply with increasing temperature. The calculated values for the concentration–concentration correlation function are lower than those corresponding to an ideal mixture and exhibit a minimum in the middle of the concentration range. The excess enthalpy data have been obtained at 4.00 MPa and 298.15 and 318.15 K by means of a high-pressure flow calorimeter. Values for the excess enthalpies are very exothermic and in agreement with those calculated from the excess Gibbs energies. The surface tension has been measured at 298.15 K. Values for the relative surface adsorption calculated from the surface tension and the chemical potentials indicate that the surface is significantly enriched in propanal for mixtures rich in methanol. A complex composition dependence of the concentration profiles can be inferred. The lattice-fluid and the lattice-fluid associated solution models have been used to describe the bulk properties of the methanol–propanal mixtures.