The hydrogen spin−lattice relaxation time in the rotating frame was measured for three whole asphalts at temperatures ranging from 20 to −45 °C. These data were used to calculate the apparent activation energies for the molecular motion of the aromatic and aliphatic components found in asphalt. The measured activation energies ranged from 8.8 to 9.8 kJ/mol for the aliphatic components in the three asphalts and were attributed to rapid methyl rotation of the terminal and branched methyl groups on the long carbon chainlength alkanes. The apparent activation energies measured for the molecular motion of the aromatic components in the three asphalts ranged from 6.5 to 7.2 kJ/mol. The low barrier to molecular motion observed for the aromatic constituents can be explained by two mechanisms. The first mechanism is spin−diffusion interaction of the aromatic ring hydrogens with the aliphatic hydrogens of the rapidly rotating methyl substituents on aromatic rings. The second mechanism is the in-plane rotation of re...