Experiments for quantifying the amplitudes of motion of methyl-containing side chains are presented that exploit the rich network of cross-correlated spin relaxation interactions between intra-methyl dipoles in highly deuterated, selectively (CH2D)-C-13- or (CH3)-C-13-labeled proteins. In particular, the experiments measure spin relaxation rates of degenerate H-1 transitions in methyl groups that, for high-molecular-weight proteins, are very simply related to methyl three-fold symmetry axis order parameters. The methodology presented is applied to studies of dynamics in a pair of systems, including the 7.5-kDa protein L and the 82-kDa enzyme malate synthase G. Good agreement between H-1- and H-2-derived measures of side-chain order are obtained on highly deuterated proteins with correlation times exceeding approximately 10 ns (correlation coefficients greater than 0.95). Although H-2- and C-13-derived measures of side-chain dynamics are still preferred, the present work underscores the potential of using H-1 relaxation for semiquantitative estimates of methyl side-chain flexibility, while the high level of consistency between the different spin probes of motion establishes the reliability of the dynamics parameters.