Catalytic behaviors of various Keggin- and Dawson-type heteropolyacids for the Michael addition of alcohols to cyclohexenone have been investigated and the results were compared with those of other acid catalysts. The heteropolyacids, particularly H 3PW 12O 40 and H 4SiW 12O 40, showed much higher catalytic activities than Cs 2.5H 0.5PW 12O 40, H-ZSM-5, and SO 4 2−/ZrO 2 and high selectivities (ca. 100%). Quantitative analysis revealed that both cyclohexenone and alcohol molecules were absorbed in the catalyst bulk phase at the early stage of the reaction and their amounts remained nearly constant during the reaction. Furthermore, the reaction rates of various heteropolyacids were approximately in proportion to the quantity of reactants absorbed in the catalyst phase, showing that the reaction proceeded in the pseudoliquid phase. The catalyst bulk phase gradually turned from a solid to a very viscous liquid lump, but this was called pseudoliquid in the present work from its appearance and the quantities of absorbed (or dissolved) molecules.