The thermodynamic properties of hydride and phenyl groups on the surface of amorphous silicon dioxide are investigated in the presented work. The characteristics of the surface silane centers (equivalent toSi-H) are determined from the data obtained by infrared spectroscopy and caloric measurements. The conversions of hydrogen and benzene with the surface are described by thermodynamic calculations at reactions take place in the gaseous phase.To model the reaction between hydrogen and the surface the thermodynamic data for (OH)(4-n)Si-n (n = 0-4) in the gaseous phase are used. The surface groups and the model molecules are comparable because the thermodynamic characteristics depend only from the local environment.The thermodynamic properties of (OH)(3)Si-C6H5 in the gaseous phase are determined to describe the reaction between benzene and the surface. The predications of these calculations are confirmed by the spectroscopic results. The properties of the surface phenyl groups (equivalent toSi-C6H5) are concluded from these data. (C) 2003 Elsevier Science B.V. All rights reserved.