The rate constants of the alkaline hydrolysis of 2-methylpropionic acid ethyl ester, alpha-hydroxy, alpha-bromo, alpha-cyano, and alpha-nitro-2-methylpropionic acid ethyl ester have been measured in water between 5-degrees-C and 45-degrees-C with the help of an appropriate conductance bridge developed in our laboratory. The influence of the polar substituents on the rate constant, a powerful probe for discussing two-particle interactions in solutions, is characterized by an intramolecular structure parameter of the activated complex and a dielectric parameter of its surroundings. The dependence of the reaction rate of the structure parameter is compared to that of substituted acetic acid derivatives. The smaller dielectric parameter of 2-methylpropionic acid esters reveals the increasing hydrophobic solvation in comparison to acetic acid derivates.