The process of isothermal crystallization and phase separation in poly(tetramethylene isophthalate) (PTMI) and multiblock copolymers of PTMI with poly(tetramethylene oxide) (PTMO) is examined by differential scanning calorimetry. Morphology development is studied from the supercooled liquid to the completion of crystallization for compositions ranging from 30 to 100 wt% PTMI over a wide range of isothermal crystallization temperatures. A model is suggested whereby phase separation in the copolymers proceeds in a manner similar to crystallization of the homopolymer, implying phase-mixed non-crystalline regions with inhomogeneous mobilities. The experimentally observed multiple endotherm behaviour in the PTMI/PTMO copolymers is a characteristic of the hard segment, and is also observed in the PTMI homopolymer.