Microcellular polypropylene structures were obtained through liquid-liquid thermally induced phase separation followed by polymer crystallization using the model system isotactic polypropylene (iPP)-diphenyl ether (DPE). The effect of the phase separation temperature and initial polymer concentration on the final morphology has been investigated. The observed morphological characteristics have been explained on the basis of the kinetics of liquid-liquid phase separation and polymer crystallization using a phase diagram developed for the iPP-DPE system, optical microscopy and scanning electron microscopy. Movement of the diluent-rich droplets (formed via liquid--liquid phase separation) due to spherulitic growth has been observed and the effect of this displacement on the final morphology has been assessed.