Hybrid combinations of sawdust and surface-treated glass fiber were used as reinforcing filler for polystyrene composites. Under study was the effect of various parameters [e.g., wood species (e.g., hardwood aspen and softwood spruce), pretreatment of sawdust (e.g., coating with polymer + isocyanate or with a silane coupling agent and grafting), filler loading, composition of sawdust: glass fiber, as well as grades of polystyrene] on the mechanical properties of the composites. The extent of improvement in mechanical properties depends on each of the variables. Treated sawdust/surface-treated glass-filled composites offered the best balance of mechanical properties compared to those of non-treated composites.