alpha-Amylases from Bacillus subtilis and Bacillus licheniformis, which has high thermostability, were adsorbed on ultrafine silica particles at various temperatures, The activities of adsorbed alpha-amylases were measured using corn starch as a substrate. In addition, the circular dichroism spectra of adsorbed alpha-amylases were measured, and the alpha-helix contents were calculated to discuss the extent of,conformational changes quantitatively, The extent of activity reductions upon adsorption of B. subtilis alpha-amylase on the ultrafine silica particles was correlated closely with that of conformational changes, and both of them were significantly increased by raising temperature from 4 to 40 degrees C. Moreover, they showed stronger temperature dependences at lower adsorption amount and pH. On the other hand, the extent of activity reductions and conformational changes upon adsorption of B, licheniformis alpha-amylase on the ultrafine silica particles was much smaller and was not affected by temperature up to 40 degrees C, indicating the strong influence of stability of enzymes on their molecular states on the solid surfaces, The mechanism of thermal deactivation of alpha-amylases upon adsorption was clearly explained by the direct observation of their conformational states on the solid surfaces. It should be noted that the thermal unfolding of adsorbed enzymes takes place in a much lower temperature range than that of free ones and is strongly affected by the adsorption conditions.