The addition of zinc borate alone or together with a hindered amine light stabilizer and ultra-violet light absorber to organic materials such as thermoplastic polyolefin resin, as is common in wood-plastic composites, provides a substantial reduction in the ultra-violet light degradation of the resin in comparison with similar materials made without zinc borate. This could provide substantial improvements in the long term durability of wood-plastic composites and other organic materials which are used in exterior environments and subject to ultra-violet exposure from the sun.