Polymerization of 3-methylidene phthalide in media of various organic solvents was studied. The stereochemical composition of macromolecules of polymethylidene phthalide (PMP) formed under different conditions of methylidene phthalide (MP) polymerization was determined by 1H and 13C NMR spectroscopy using 2D NOESY, HSQC, and COSY procedures. According to the X-ray diffraction data, no substantial differences are observed in packing of PMP macromolecules with different ratios of syndio-or isotriads: the predominant feature of the X-ray diffraction patterns is the presence of two intense amorphous halos. The diffraction patterns of a series of PMP films cast from solutions contain rather narrow low-intensity peaks probably caused by the formation of a small amount of crystalline microdomains.