Technical Abstract: Phenolic phytochemicals are known to perform several functions ranging from phytoprotectants, to protecting lipids in food products, to antioxidant activity in animals and humans. The need for phenolic phytochemical reference material is critical for developing robust validated assay procedures for accurately quantifying antioxidants in various matrices as wide array of single purified reference phenolic compounds have been used by different research groups. Five compound mixture (caffeic acid, morin hydrate, hesperetin, catechin hydrate, and epigallocatechin gallate) containing phenolic compounds from four subgroups (phenolic acid, flavone, flavanone, and flavan-3-ol) was prepared. The mixture was assayed for stability evaluation by high performance liquid chromatography using a diode-array detection procedure for a three-month time interval. The HPLC analysis result showed that there was no significant interaction between different components of the mixture. The among-sample relative standard deviation of all five phenolic compounds as well as the total HPLC area was below one percent. The relative standard deviation due to instrument was below two percent and the total relative standard deviation among-days was below five percent. The above results unambiguously suggest that sample was stable for a three-month time interval in amber vial stored in a refrigerator below 5oC. This mixture is currently being used for the single laboratory validation study for the assay of total phenolic content by Folin-Ciocalteu method and the antioxidant capacity by oxygen radical absorbing capacity procedure.