12-Tungstophosphoric acid and its acidic salts, H3PW12O40, CsxH3-xPW12O40 (abbreviated as Cex; x=0.33, 2.23, 2.5, 2.7, and 3), BaxH3-2xPW12O40 (abbreviated as Bax; x=0.5, 1.25, and 1.5), and CexH3-3xPW12O40 (abbreviated as Cex; x=0.33, 0.66, 0.87, and 1), were applied as acid catalysts to three organic reactions which are very different in the polarities of substrates and solvents: (1) decomposition of cyclohexyl acetate (in m-xylene), (2) esterification of benzoic acid by l-butanol (without solvent), and (3) rearrangement of benzopinacol tin toluene). It was demonstrated that these three reactions represent three typical reaction fields in the acid-catalyzed reactions of heteropoly compounds in the liquid phase. Reaction (1) proceeded on the surface of solid catalysts (a surface-type reaction). In this case, the catalytic activities were primarily determined by the surface acidity. Reaction (2) mostly took place in the homogeneous solution and the catalytic activities were correlated with the amount of the heteropolyacids dissolved in the solvents. Catalysis in pseudoliquid phase was observed for reaction (3), where the catalytic activities depended on the amount of substrates absorbed in the solid bulk of the catalysts. Thus, heteropolyacid catalysts provide three different reaction fields, depending on the polarity of substrate and solvent. The orders of the catalytic activity very much differed reflecting the reaction field. These results demonstrate that the recognition of these reaction fields is important for the understanding and the design of heteropolyacid catalysis in the liquid phase.