Optical microscopy, differential scanning calorimetry, and small-angle X-ray scattering techniques were used to study the influence of crystallization conditions on the morphology and thermal behavior of samples of ternary blends constituted by isotactic polypropylene (iPP), atactic polystyrene (aPS), and a novel graft copolymer of unsaturated propylene with styrene (uPP-g-PS) with the purpose of assessing the uPP-g-PS capability to act as a compatibilizer for iPP/aPS materials. It was shown that the presence of the uPP-g-PS copolymer affects the interfacial tension between the IPP and aPS phases in the melt state, with the aPS particle size and the particle-size distribution being, in fact, strongly modified. In samples of iPP/aPS/uPP-g-PS blends, isothermally crystallized from the melt at a relatively low undercooling in a range of the crystallization temperature of the iPP phase, the addition of the uPP-g-PS copolymer induced a drastic change both in the aPS mode and the state of dispersion and in the iPP spherulitic texture and inner structure of the spherulite fibrils. In particular, the phase structure developed in the iPP/aPS/uPP-g-PS materials was characterized by a crystalline lamellar thickness of the iPP phase comparable to that shown by the plain iPP. The extent of the induced modifications, that is, the degree of compatibilization achieved, resulted in a combined effect of composition and undercooling. Also, relevant thermodynamic parameters of the iPP phase, such as the equilibrium melting temperature (T-m) and the folding surface free energy (sigma(e)) of the lamellar crystals, were found to be influenced by the presence of the uPP-g-PS copolymer. A linear decrease of the T-m and sigma(e) values with increasing uPP-g-PS content was, in fact, observed. Such results have been accounted for by an increase of the presence of defects along the iPP crystallizable sequences and by the very irregular and perturbed surface of the crystals with increasing copolymer content. The observed decrease in T-m values revealed, moreover, that, in the iPP/aPS/uPP-g-PS blends, the iPP crystal growth occurs under comparatively lower undercooling, in line with higher crystalline lamellar thickness shown by SAXS investigation.