Four crystalline forms of isotactic poly(4-methyl-1-pentene) (i-P4MP) (forms I, II, II, and IV) have been studied through solid-state C-13 NMR CP-MAS spectroscopy. The assignment of the resonances observed in the C-13 NMR CP-MAS spectra was made by dipolar dephasing experiments. The presence in all spectra of two distinct resonances, separated by approximate to 3 ppm, of the methyl carbons indicates that the two methyls belonging to the monomeric unit are nonequivalent. This has been explained on the basis of the gamma-gauche shielding interaction and of the conformations of polymer chains and lateral groups. The resonances observed in the spectrum of form III are markedly deshielded compared to those of other forms, whereas the resonances of form I are deshielded compared to those of form IV. This deshielding has been consistently interpreted on the hypothesis that the amount of the gamma-gauche shielding effect depends on the value of the torsion angles of the main chain. Indeed, the torsion angles of the main chain for the 7/2 helical conformation of form I deviate, on average, by 12-15 degrees from the exact staggered values of 60 degrees and 180 degrees (the average values of the torsion angles are theta(1) = 72 degrees and theta(2) = 195 degrees) they deviate by 22-26 degrees for the 4/1 helical conformation of form III (theta(1) = 82 degrees and theta(2) = 206 degrees). The gamma-gauche shielding parameter, normally -5 ppm when the gauche angle is nearly 60 degrees, decreases to a value between -2 and -2.5 ppm when the torsion angle deviates by nearly 20 degrees from 60 degrees. In the form IV, the upfield shift of the resonances, compared to forms I and III indicates that the carbon atoms experience a full gamma-gauche shielding interaction and, hence, the main chain torsion angles theta(1) and theta(2) are near 60 and 180 degrees, respectively, typical of a helical 3/1 conformation. This analysis allows us to suggest that the form IV of i-P4MP is characterized by chains in 3/1 helical conformation. The resonances of the methyl carbons of the four forms of i-P4MP show narrow splittings (1-2 ppm) which should be traced back to packing effects. This has provided a chance of testing the packing model of form I and form III proposed in the literature. In the spectrum of form I, the resonance of backbone methine carbons is also split in a doublet. This splitting is due to conformational effects, rather than packing effects and confirms that, according to the refinement of the crystal structure of form I, the chains assume a helical conformation slightly distorted from the uniform 7/2 helix. The succession of torsion angles along the main chain is ...TGT'G " T " G "..., instead of (TG)(n). Therefore, backbone methine carbons of different monomeric units are conformationally nonequivalent because they experience different amounts of gamma-gauche shielding effects owing to different values of the gauche torsion angle theta(1) of different monomeric units.