The preparation of polymethylvinylsiloxane rubbers with silica (SC) and black rice husk ash (BRHA) as fillers is reported. We conducted stress-strain experiments on these vulcanized rubbers to study the reinforcement properties of the fillers. Curves showing the dependence of the stress on the reciprocal of the elongation ratio displayed, in most cases, a slow upturn starting at rather low values of the elongation ratio attributed to a rather wide distribution of chain lengths between crosslinking points. Physical crosslinks between the hydroxyl groups of SC fillers and the polymer matrix seemed to enhance the modulus. BRHA-polymer interactions were rather weak in comparison with those occurring between SC and the polymer, presumably as a consequence of the carbon coating of the surface of the former fillers. These interactions were even less important for calcined BRHA. The absence of voids in the polymer-filler interfaces was proven by the analysis of gas diffusion across the rubbers. (C) 2003 Wiley Periodicals, Inc.