Esterification of acrylic acid with l-butanol has been studied in a heterogeneous solid-liquid reaction system using a variety of solid acids including an acidic salt of H3PW12O40, Cs2.5H0.5PW12O40 The catalytic activity (per proton) of liquid acids such as heteropolyacids, H2SO4, and para-toluenesulfonic acid increased as the acid strength of the solution increased, but the difference in the activity among them was not significant. In the solid-liquid reaction system, Cs2.5H0.5PW12O40 exhibited the highest catalytic activity in the unit of catalyst weight among the solid oxide catalysts, while the activity of Cs2.5H0.5PW12O40 was less than those of organic resins such as Nafion and Amberlyst 15. Contrary to the liquid acids, the specific activity values (per proton) of solid acids were significantly different. It was demonstrated that the activity of Cs2.5H0.5PW12O40 was almost retained after the addition of water, while the activities of the organic resins greatly decreased. The water-tolerant property of Cs2.5H0.5PW12O40 is presumed to be due to the hydrophobic nature of the surface.