Trimethylphosphine oxide (TMPO) molecules were adsorbed on H-type mordenite to probe the surface acidity by solid-state NMR. We found that dichloromethane was coadsorbed when it was used as a solvent. To avoid the effect of the solvent, TMPO was adsorbed from the gas phase, and we have obtained H-1 and P-31 magic-angle-spinning (MAS) NMR spectra of TMPO-adsorbed H-type mordenite without the effect of the solvent.