Cross Polarization Magic Angle Spinning (CPMAS) 13C-NMR spectroscopy is a powerful tool to investigate structure and conformational behaviour of humic substances. In the present study, a soil humic acid was subjected to preparative High Performance Size Exclusion Chromatography (HPSEC). Eleven size-fractions were separated and analyzed by CPMAS 13C-NMR spectroscopy. The spectra showed that the largest molecular size fractions contained aromatic, aliphatic and carbohydrate components. Carbohydrate content and length of alkyl chains appeared to decrease with decrease of molecular sizes. A progressive reduction of aromatic carbons was also observed with decrease of molecular sizes of separated fractions. Measurements of cross polarization (T_CH) and proton spin lattice relaxation (T_1ρ(H)) times indicated that the eleven size-fractions could be encompassed in two main conformational groups. A first group with nominal molecular sizes above 100 kDa, revealed slower molecular motions than the group of size fractions having molecular sizes <100 kDa. The NMR spectra, thus, indicated that conformational rigidity and low molecular motion can be associated to large nominal molecular size of humic matter.