Silanol groups that are present on the surface of rice hull ash or silica ash fillers can positively influence the reinforcing character of the filler. However, being hydrophilic, they present the problem of aggregation and moisture absorption. Physicochemical contributions to reinforcement, provided by silanol groups, influence the filler-polymer and filler-filler interactions. In this study, we investigated the surface energetics of silica ash particles and the effect of chemical surface modification on the free surface energy of silica ash particles was studied. Inverse Gas Chromatography (IGC), a successful evaluation technique for the surface energy of solids, has been used at infinite dilution to determine the nature of surface interaction of various probes with a silica ash surface. The Gibbs free energies and enthalpies of specific interactions were also determined to estimate the acid-base characteristics of the surface of silica ash particles. The influence of heat treatment and surface modification upon the physicochemical parameters was also investigated. It was found that the silica ash surface is acidic in nature. Modification of the surface, by hydrophobization using silane agents, reduces the specific component of surface energy and could be expected to positively affect the reinforcing character. Surface modification reduced the specific component of surface energy by 80% and positively affects the filler-matrix interaction. (c) 2005 Springer Science + Business Media, Inc.