Dehydration of ethanol in the psuedoliquid phase of 12-tungestophosphoric acid, H{sub 3}PW{sub 12}O{sub 40} was studied by means of solid-state NMR combined with IR, thermal desorption, and transient-response method. It was confirmed by the transient-response method using isotopically labeled ethanol that a large amount of ethanol was absorbed in the lattice of the catalyst bulk (psuedoliquid phase) under the reaction conditions, and the reaction proceeded in the bulk phase. Probable reaction intermediates of the ethanol dehydration such as protonated ethanol dimer ((C{sub 2}H{sub 5}OH){sub 2}H{sup +}) and monomer (C{sub 2}H{sub 5}OH{sub 2}{sup +}) were directly identified in this phase by solid-state NMR spectroscopy. The assignments were supported by FIR spectroscopy and stoichiometry of ethanol absorption. By comparison of the NMR data with the results of thermal desorption of ethanol, the dimer and monomer species are very likely the intermediates for diethyl ether and ethylene formation, respectively. The unusual pressure dependence observed for the dehydration of ethanol was reasonably explained by charges in the concentration of these intermediates in the psuedoliquid phase. 46 refs., 11 fig., 3 tab.