Monte Carlo molecular simulations are reported to investigate the amorphous region between crystalline lamellae of isotactic polypropylene (iPP). It is the first such study in which consideration is given to the nature of pendant side groups on the polymer chain. The crystalline lamellae are modeled after the alpha(2) crystal of iPP, while the interphase and amorphous domains are equilibrated with respect to both spatial and topological degrees of freedom. A united atom force field is used to describe the interactions between the methyl, methylene, and methyne moieties that constitute a polypropylene molecule. Mass density and bond orientational order profiles, distributions of chain populations, interphase thicknesses, interfacial energies, local conformations as characterized by torsional angle sequences, and chain reentry statistics are studied for three different temperatures. Our results match well with the available experimental data and provide a molecular level understanding of the semicrystalline interphase for isotactic polypropylene. A comparison with linear polyethylene (PE) reveals the effects of chemical architecture on the crystal-amorphous interphase in semicrystalline polymers.