Stabilizer compositions having a stabilizing amount of at least one co-active agent; an ultraviolet light absorber chosen from an ortho-hydroxy benzophenone, and/or an ortho-hydroxyphenyl benzotriazole; and a hindered amine light stabilizer, are provided herein, along with masterbatch concentrates containing same, and processes for using same for stabilizing polymeric organic materials to protect against light and thermal degradation due to exposure to UV irradiation.