The interlayer silylation of a layered silicate H-RUB-18 (Si4O7(OH)2) using a new aromatic silylating reagent containing a phosphonic acid group (4-phosphonophenylsilane: PPS) was demonstrated (H-PPS-RUB-18). The phosphonic acid groups were attached to the silicate layers through the reaction of H-RUB-18 with (4-diethoxyphosphorylphenyl)-triethoxysilane (p-PPS-E), and the ester moieties were subsequently hydrolyzed with hydrochloric acid. H-PPS-RUB-18 is a solid acid, as indicated by the intercalation of various alkylamines and the catalytic acetalization of ketones. A systematic increase in interlayer spacing leading to surface acidic properties was obtained through intercalation with a series of alkylamines. In addition, H-PPS-RUB-18 was exfoliated, resulting in single-layer nanosheets with ca. 2.0 nm thickness. The catalytic acetalization of ketones was related to the interlayer spacing of the modified RUB-18.