The enthalpies of combustion of o-, m-, and p-chlorobenzoic acid have been determined in an adiabatic rotating-bomb calorimeter. The enthalpies of formation have been obtained by combination of the experimental data with the accepted values for the enthalpies of formation of water, carbon dioxide, and aqueous hydrochloric acid. The results of other investigators are discussed briefly. The resulting values and their estimated 95 percent confidence limits are as follows: o-Chlorobenzoic acidm-Chlorobenzoic acidp-Chlorobenzoic acid ΔHc° (25 °C)-3087.91 ± 0.69 kJ/mol-3068.05 ± 1.53 kJ/mol-3064.40 ± 0.66 kJ/molΔHf° (25 °C)-404.83 ± 0.74 kJ/mol-424.59 ± 1.55 kJ/mol-428.16 ± 0.72 kJ/mol. Where ΔHc° corresponds to the process: -404.61-424.37-429.94C7H5O2Cl(c) + 7 O2(g) + 198 H2O(liq) → 7 CO2(g) + [HC1 + 200 H2O](liq).