The heat capacities, C-p, of trimethyl phosphite, 2-dimethylphosphonomethylbenzonitrile, and 2-chloromethylbenzonitrile are measured with an adiabatic calorimeter, and the standard enthalpies of formation, Delta H-f(o)(298.15 K) (l or cr), for the three substances are determined. For this purpose, C-p(T) values of trimethyl phosphite and 2-dimethylphosphonomethylbenzonitrile are measured in the temperature range from (298.15 to 350.32) K, and the C-p(T) curves of 2-chloromethylbenzonitrile are also obtained for the crystal (300.35 to 329.00 K) and the liquid (337.26 to 372.45 K), respectively. The deviation of experimental heat capacities from the corresponding smoothed values lies within +/- 1.5 %. A fusion transition phenomenon is found from the C-p(T) curves of 2-chloromethylbenzonitrile, and the enthalpy of fusion is measured with a differential scanning calorimeter (DSC). The gas-phase standard enthalpies of formation, Delta H-f(o)(g, 298.15 K), of the three compounds are evaluated by the Benson group additive method. The critical temperature T-c, critical pressure P-c, and enthalpy of vaporization Delta H-vap(o) of the compounds are also calculated by different group additive methods. On the basis of the above data, the values of Delta H-f(o)(l, 298.15 K) of trimethyl phosphate and 2-dimethylphosphonomethylbenzonitrile and of Delta H-f(o)(cr, 298.15 K) of 2-chloromethylbenzonitrile are estimated. A thermochemical cycle including a chemical reaction is designed to examine the reliability of the values by measuring the heat of reaction. The result shows that the enthalpies of formation, Delta H-f(o)(298.15 K) (l or cr), for the three substances are acceptable.