A vapour-liquid equilibrium study of the 4-chlorobenzene sulphonic acid-water system has been carried out at atmospheric and reduced pressure, and the 100 degrees C isothermal VLE diagram derived. The vapour pressure of ClC6H4SO3H . 0.5H(2)O was measured at 100-130 degrees C and provides improved definition of the liquid curve at very high acid concentration. There is evidence of strong acid-water association in this range. The enthalpies of solution of crystalline anhydrous and hydrated 4-chlorobenzene sulphonic acid in water at 30 degrees C have been measured by isoperibol calorimetry and the enthalpy of fusion of the anhydrous acid by differential scanning calorimetry. The enthalpy of solution is strongly dependent on the degree of hydration but for the crystalline and liquid anhydrous acid, Delta(sol)H = -22.2 +/- 1.0 and -33 +/- 2 kJ mol(-1) respectively. Hydrate association energies are derived. The results are assessed in relation to other types of acids.