The solid acidity of the interlayer aluminol surfaces of kaolinite was explored by solid-state 31P nuclear magnetic resonance with magic angle spinning (MAS) using triethylphosphine oxide (TEPO), which formed a monolayer with a uniform orientation between the layers of kaolinite as a probe molecule. Intercalation of TEPO between the layers of kaolinite was achieved using methoxy-modified kaolinite as an intermediate. The presence of TEPO in the reaction products was revealed by the two signals at 21 and 7 ppm, which were assignable to ethyl groups in TEPO, in the solid-state 13C nuclear magnetic resonance with cross polarization and magic angle spinning techniques (13C CP/MAS NMR). The presence of TEPO between the layers of kaolinite was demonstrated by the expansion of basal spacing from 0.86 nm, the interlayer distance of methoxy-modified kaolinite to 1.16 nm, as shown by the X-ray diffraction patterns, suggesting the formation of a TEPO monolayer between the layers of kaolinite. The formation of hydrogen bonds between the P=O groups of TEPO and the aluminol groups on the interlayer surfaces of kaolinite was also revealed by the appearance of an additional OH stretching band at 3598 cm-1 in the Fourier-transform infrared spectrum and narrow solid-state 31P MAS NMR signals observed at 55-53 ppm which were shifted from the position of the physisorbed TEPO (50 ppm). These results clearly indicate that the solid acidity of interlayer aluminol groups of methoxy-modified kaolinite was probed using an interacted TEPO monolayer.