Fibers, made from polyesters of β-hydroxyethoxy aromatic acids, and their copolyester, and those from polyethylen terephthalate (PET), were treated with aqueous caustic soda solutions.Rate constants of solution, k (g/hr/cm2), are calculated from the equationwhere x is the loss of weight by solution, d0 (cm) the initial diameter of a fiber, t (hr) the treatment time, and ρ (g/cm3) the density of the fiber.At room temperature, k of fibers from polyester A and coplyester are much smaller than those from polyethylen terephthalate, but higher the temperature, they become closer. Draw ratio and heat-treating temperature have some influence on k, but chemical constitution of polyesters have much more.We conclude that k, rate constants of solution, are most influenced by the rate of hydrolysis of ester combination of the molecular chains on the surface of fibers. The differences of k, caused by chemical constitution of polyesters, may be explained by Hammet's Law.