Such metal oxide as SO42-, MoO3, WO3, and V2O5 spread readily on supports like SnO2, ZrO2, and TiO2 due to the different properties between acid and base oxides to generate the acid site on the monolayer. Number, strength, and structure of the acid site were characterized by temperature-programmed desorption (TPD) of ammonia principally, together with various physico-chemical techniques, and its role for catalytic reactions was studied. Approximately, one to two acid sites were stabilized on 1 nm(2) of the surface, which consisted of four to eight metal atoms. The limit in surface acid site density was estimated on the monolayer based on the concept of solid acidity on zeolites. Sequence of the metal oxide to show the strong acidity was, SO42- > WO3 > MoO3 > V2O5, and for the support oxide to accommodate the monolayer, SnO2 > ZrO2 > TiO2 > Al2O3. From these combinations, the metal oxide monolayer to show the adequate strength of acid site could be selected. Bronsted acidity was observed often, however, the Lewis acidity was prevailing on the reduced vanadium oxide. The structure of acid site, Bronsted or Lewis acid site, thus depended on the oxidation state. Relationship of the profile of solid acidity with various catalytic activities was explained. The solid acid site on the monolayer will possibly be applied to environment friendly technologies. (C) 2003 Elsevier B.V. All rights reserved.