A polymer optical fiber is provided which shows improved hydrolytic stability. This fiber comprises a polymeric optical core and cladding layer, surrounded by a polymeric overcladding layer which comprises a miscible blend of one or more hydrolytically stable amorphous polymers. By varying the ratios of the component polymers in the overcladding blend, the glass transition temperature and the coefficient of thermal expansion of the overcladding layer may be tuned to optimize the attenuation and bandwidth of the plastic optical fiber.