The effect of fiber annealing on the optical and mechanical properties of glass-fiber-reinforced poly(methyl methacrylate) (PMMA) composites has been studied. Annealing glass fibers at different temperatures can slightly change their refractive index, thereby changing the temperature where the optical transmission of the composites is a maximum. An explanation is given for the increase in maximum transmission which occurred for composites containing annealed glass fibers. Annealing the glass fibers is a means of fine-tuning the optical properties of the composites. The mechanical properties of the composite are superior to PMMA for fibers annealed below 500°C. © 1994.