The structure development in melt crystallized polypropylene (PP) containing different types and concentration of calcium sulphate has been studied by X-ray diffraction and optical polarizing microscopy. The commercial CaSO4 containing mostly the anhydrite form gives the α form of PP while the CaSO4 prepared by in situ method yields the β form of PP. This takes place first by conversion of the dihydrate form of CaSO4 to the τ form which preferentially nucleates the β form of PP. The crystallization behaviour in the latter case clearly reveals two processes. The presence of PEO which is used for the in situ preparation of CaSO4, however, is found to hinder the rate of crystallization and nucleation of the β form of PP but it gives a high degree of orientation of the CaSO4 needle-like crystals during extrusion of the PP-CaSO4 composites. © 1998 Chapman & Hall.