Carbon-13 and silicon-29 spin-lattice relaxation times and the corresponding nuclear Overhauser enhancements have been measured for PhnXH4-n(n= 1-4,X= C or Si). Thence dipolar and other contributions to the relaxation rate have been separated. The dipolar contribution is dominant, except for -CH3and -SiH3groups. Correlation times for overall molecular motion have been estimated, andT1ratios for phenyl groups have been used to compare the rotational mobility of the phenyl groups. Internal molecular motion is more hindered in phenylmethanes than in phenylsilanes. Results for diphenylmethane and diphenylsilane are consistent with strong coupling between internal and overall molecular motion.