On acid surfaces, trimethylphosphine (TMP) forms either Bronsted or Lewis site adducts, TMPH(+) or TMP:L, respectively. In TMPH(+), the P-31 magnetization decay is practically monoexponential and the P-31 spin-lattice relaxation rate (1/T-1) is not thermally activated. In TMP:L, the magnetization decay is biexponential and the relaxation rate is thermally activated. Using the known internuclear distances in TMPH or TMPH(+), it is shown that the relaxation rates can be accounted for by using the homo- and heteronuclear (intramolecular) contribution rates. The only adjustable parameter is the preexponential factor in the correlation time (tau(0)). tau(0) is 10(-11) s for the PH vector reorientation and 10-(13) s for the H-H (methyl) vector rotation in TMP:L. It is 10(-7) s for the PHC reorientation in TMPH(+). It is observed that the resonance frequency in TMPH(+) or TMP:L is temperature independent, ruling out chemical exchange. In one case only, (TMPH(+) on dealuminated H-Y zeolite) a change in the isotropic chemical shift with temperature has been observed.