A C-13 spin-lattice relaxation study at 298 K of substituted adamantanes in the liquid state at constant (0.10 M) concentration in deuterated methylene chloride and in the solid state has been conducted. The anisotropic rotational diffusion tensor elements have been determined from the C-13 T1 data. Nearly spherical adamantane undergoes isotropic rotational diffusion with R1 = R2 = R3 = 11.0 X 10(10) rad2/S, while 1-adamantanol and 1-adamantyl isocyanate undergo axially symmetric rotational diffusion with R1 = R2 = 4.3 X 10(10) rad2/s and R3 = 10.6 X 10(10) rad2/s, and R1 = R2 = 3.1 X 10(10) rad2/s and R3 = 11.2 X 10(10) rad2/s, respectively. 2-Adamantanone undergoes asymmetric rotational diffusion with R1 = 1.0 X 10(10) rad2/s, R2 = 2.0 X 10(10) rad2/s, and R3 = 24.0 X 10(10) rad2/S. In the solid state at 294 K, adamantane’s rotational diffusion is still isotropic, but is 10 times slower, and that of 1-aminoadamantane is axially symmetric with R1 = R2 = 0.59 X 10(10) rad2/s and R3 = 2.72 X 10(10) rad2/s. In all cases, the intramolecular heteronuclear dipolar interaction is the main relaxation mechanism for C-13.