Heteropoly acids and their related compounds function not only as acidic solid catalysts with redox characteristics, but also as catalysts selective to the oxidation reaction lik the formation of methacrylic acid from methacrolein. For the investigation of their catalytic characteristics, the isomerization of n-butenes was studied using the catalysts of 12-molybdo phosphoric acid and its several metal salts. The catalytic activities of heteropoly acid were increased with the injected water vapor, and metal salts prepared with partial exchange of hydrogen ions gave much higher activities.