Abstract: Acrylic light pipe has adequate thermal and photo-thermal stability for many purposes, but is deficient in maintaining clarity, color, and good optical properties under conditions of exposure to high temperatures, especially in combination with exposure for lengthy periods to passage of light. Improved thermal stability, as reflected in reduced color formation, can be imparted by adjusting the polymerization conditions to produce the uncured core polymer of the core/clad construction with a much reduced terminal vinyl content, preferably below 0.5 vinyl groups/1000 monomer units. This process improvement, in combination with selected addition of a combination of certain hindered phenols and hydrolytically stable organic phosphites, together produce a substantial improvement in the resistance to discoloration under photo-thermal conditions, while maintaining the resistance to discoloration under thermal conditions.