The thermal properties of a number of polyphosphazene random and block copolymers synthesized by the anionic polymerization of phosphoranimines have been examined by differential scanning calorimetry and correlated with variable-temperature wide-angle X-ray scattering. These studies show the effect of structure, molecular weight, composition, side groups and thermal history on the thermal transitions of these materials. In addition, annealing studies were performed to estimate the degree of crystallinity and attempts were made to detect microphase separation by small-angle X-ray scattering.