Natural fibers are rich in cellulose and they are a cheap, easily renewable source of fibers with the potential for polymer reinforcement. The presence of large amounts of hydroxyl groups makes natural fibers less attractive for reinforcement of polymeric materials. Composites made from polystyrene (PS)/styrene butadiene rubber (SBR) blend and treated rice husk powder (RHP) were prepared. The RHP was treated by esterification and acetylation. A similar series of composites was also prepared using maleic anhydride-polypropylene (MA-PP) as a coupling agent. The processing behavior, mechanical properties, effect of thermooxidative ageing, and surface morphology of untreated and chemically modified RHP were studied. There was a decrease in tensile strength (except MA-PP composites), elongation at break, and Young's modulus in chemically treated RHP composites. The postreaction process during thermooxidative ageing enhanced the tensile strength and Young's modulus of the esterified and MA-PP composites. Acetylation treatment was effective in reducing the percentage of water absorption in RHP/PS-SBR composites. In general chemically treated RHP/PS-SBR composites and MA-PP showed a better matrix phase and filler distribution. However, the degree of filler-matrix interaction was mainly responsible for the improvement of mechanical properties in the composites. (C) 2004 Wiley Periodicals, Inc.