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Spin−lattice relaxation times, T1, have been measured from 200−340 K for the protons in H2and HD molecules dissolved in toluene-d8and incarcerated in C60. It is found that HD relaxes more slowly than H2in both environments and at all temperatures, as expected from the smaller values of the spin-rotation and dipole−dipole coupling in HD compared to H2. More detailed analysis using models developed to describe relaxation in both condensed media and the gas phase indicates that transitions among the rotational states of H2occur at a rate similar to those of HD in both toluene-d8solution and in C60, in contrast to the situation in gas phase collisions between hydrogen and He or Ar, where the lifetimes of rotational states of HD are markedly shorter than those for H2. Measurements of the relative 1H chemical shifts of H2and HD, the coupling constant JHD, and the widths of the HD peaks at various temperatures revealed only small effects with insufficient accuracy to warrant more detailed interpretation. [ABSTRACT FROM AUTHOR]