Thermal characterization has been carried out on a series of biphenol- and hydroquinone-based novel poly(arylene ether ether sulfide) polymers. Thermogravimetric analyses of the sulfide systems indicate high thermal stability in air as well as in nitrogen. X-ray studies showed the poly(ether ether sulfoxide) precursor materials to be amorphous and the poly(ether ether sulfide) materials to be semicrystalline. Based on the number-average molecular weight of the poly(ether ether sulfoxide) materials, and the glass transition temperatures determined by differential scanning calorimetry (d.s.c.), the limiting T-g infinity has been determined for both amorphous precursors. The biphenol-based precursor sulfoxide had a T-g infinity of 214 degrees C and the hydroquinone-based precursor had a T-g infinity of 179 degrees C. D.s.c. studies showed the biphenol-based poly(ether ether sulfide) to have a T-g of 142 degrees C and T-m of 347 degrees C, and the hydroquinone-based poly(ether ether sulfide) to have a T-g of 100 degrees C and a T-m of 243 degrees C. The equilibrium melting temperature T-m degrees has been determined for both sulfide polymers using the Hoffman-Weeks method. The hydroquinone-based poly(ether ether sulfide) exhibited a T-m degrees of 278 degrees C, and the biphenol-based poly(ether ether sulfide) showed a T-m degrees of 371 degrees C. The effect of time and temperature in the melt on the subsequent crystallization behaviour of the two sulfide materials has also been investigated as a function of molecular weight.