The longitudinal proton relaxation time T1 in methyl naphthalene crystals, differing in the arrangement and number of the substituted CH3 groups, was measured as a function of the temperature above 77 °K and the magnetic field between 0.9 and 20 kOe. The results can be described by hindered rotation of the methyl groups with the jumping times and activation energies strongly dependent on the group arrangement. In the β-position the rotational barrier of 0.8 kcal/mol is predominantly determined by the infermolecular interaction, whereas in the case of the a-position and for adjacent CH3’s the hindering potential of 2.4 kcal/mol arises largely from the intramolecular term.