The kinetics of crystallization have been studied for a series of novel poly(ether ether sulfide)s based on a biphenyl moiety in the backbone (M-n = 14.3K, 19.1K), referred to as biphenyl sulfide (T-g = 142 degrees C, T-m = 347 degrees C) and phenyl moieties in the backbone (M-n = 8.1K, 19.9K, 34K), referred to as phenyl sulfide (T-g = 100 degrees C, T-m = 243 degrees C). Isothermal melt crystallization kinetics were analyzed based on the Avrami equation. Avrami exponents close to three were obtained for the phenyl sulfides, independent of molecular weight or crystallization temperature, which implies growth of three-dimensional spherulitic superstructures following heterogeneous nucleation. For the biphenyl sulfides, values closer to 2 were obtained for the exponent, also independent of molecular weight or crystallization temperature, which could imply the incomplete development of three-dimensional superstructures following heterogeneous nucleation. Nonisothermal crystallization kinetics were also studied by cooling from the melt; in all cases studied, the Ozawa analysis could not well describe the evolution of crystallinity, probably because of the inapplicability of some of the inherent assumptions in this type of analysis. The data were analyzed using the conventional form of the Avrami equation, which yielded good fits. This semiquantitative method of analysis yields a reduced rate constant that was found to increase with increasing cooling rate and decreasing molecular weight. The results of the isothermal and nonisothermal crystallization studies carried out on the poly(ether ether sulfide)s have been compared wherever possible to the results available for PPS and PEEK.