Excess enthalpies, vapor-liquid equilibrium data and surface tensions of the highly non-ideal associated mixtures formed by methanol or ethanol and propanal have been measured in the temperature range of 288.15-318.15 K. These mixtures show negative deviations from ideality, including a negative azeotrope for the methanol-propanal system in the methanol-rich region. The values for the excess enthalpies are very exothermic and become less negative as temperature increases. Methanol-propanal mixtures exhibit minima of approximately -8.6 and -8.0 kJ mol(-1), at a mole fraction close to 0.5 and 298.15 and 318.15 K, respectively. The ethanol-propanal system exhibits minima of approximately -7.2 and -5.0 kJ mol(-1) at 298.15 and 318.15 K, respectively. 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. 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. A complex composition dependence of the concentration profiles can be inferred. The Lattice-Fluid Associated Solution, the UNIQUAC Associated Solution and the Extended Real Associated Solution models have been used to describe the bulk properties of the alcohol-propanal mixtures.