An improved methodology for the structural characterization of heavy fractions of petroleum by hydrogen-1 and carbon-13 nuclear magnetic resonance was applied to three fresh and unfractionated asphalts in solution. The results were compared with those generated by two other known characterization methods applied to the same NMR spectra. The comparison revealed inconsistencies in the traditional methods for the structural characterization of this type of materials. The improved methodology can be applied to unfractionated samples, and it provides a more detailed structural characterization of the asphalts than previously obtained. The new method provides a better correlation between physicochemical properties and the chemical structure of the complex mixtures of hydrocarbons.