Triaryl phosphites bearing electron donating and electron withdrawing substituents were prepared through the reaction of sodium phenoxides with phosphorus trichloride. The reactions were performed in a Mettler RC I reaction calorimeter. The main purpose of this work was the determination of Hammett p from the synthesis of substituted triaryl phosphites through the interpretation of calorimetric data. The phenoxide bearing a methoxide group was the most reactive, and the one bearing the nitro group was the least reactive. It was demonstrated that the reaction rate depends mainly on the addition rate of phosphorus trichloride solution. A good correlation between the Hammet parameters (sigma(p)(+)) was obtained, indicating a reaction mechanism in which a decrease of the negative charge occurs in the transition state. (c) 2004 Elsevier B.V. All rights reserved.