Trimethylphosphine reacts with protons in acidic zeolites to form [(CH3)(3)P-H](+) complexes within the cavities and channels. The mobility of the protonated adduct strongly depends on the available space, which is a function of the amount of (CH3)(3)P in the zeolite and the size of the guest molecule relative to the cavity or channel dimensions. In an H-Y zeolite containing one molecule per large cavity, the motion is sufficiently rapid to average out a large P-H dipolar interaction, such that even in a static NMR experiment the lines due to J(P-H) coupling are well resolved. The motional rate is greater than 100 kHz. By contrast, the geometric constraints imposed by the smaller channels in ZSM-5, which are comparable in size to trimethylphosphine, severely restrict the motion. The chemical shift and J coupling values were determined for H-ZSM-5 and dealuminated H-Y zeolites, which are known to be strongly acidic, and for a normal H-Y zeolite, which is less acidic. The P-31 chemical shift values were the same within experimental error, but a smaller J coupling in H-ZSM-5 is opposite from what one might expect on the basis of the extent of proton donation. The latter observation suggests that other factors, such as the radius of curvature of the cavities and channels, may play a role in acid-base interactions.