In the extraction of zinc(II) from hydrochloric acid solutions by trioctylmethylammonium chloride in various organic diluents such as benzene, chlorobenzene, o-dichlorobenzene, toluene, m -xylene, nitrobenzene, carbon tetrachloride and 2,2-dichloroethane, the heat change associated with zinc(II) extraction has been examined by calorimetry. In the case of benzene used as diluent, it is confirmed that the heat change detected can be interpretated as the summation over the change in enthalpies for the relevant steps to the extraction of zi(II), taking the change in enthalpy obtained from the temperature-dependence of the distribution coefficient into account. Applying this approach of zinc(II) extraction for other diluents, it is found that the distribution of zinc (II) between aqueous and organic phases can be explained very well by assuming a regular solution.