Excess volumes of the systems acetonitrile (ACN) + acetic acid (AA), + propionic acid (PA) and + isobutyric acid (IBA) at 298.15 and 308.15 K and + trimethylacetic acid (TMA) at 313.15 K have been measured using dilatometric method. The observed values are found to be negative for the first three systems while the last system has sigmoid concentration dependence. Excess enthalpies for the first three systems at 308.15 K and for the last system at 313.15 K have also been determined and found to be positive. The results have been explained in terms of depolymerization of the acids, dipole–dipole interactions between heteromolecules and the steric hindrance caused by the increasing methylation.