Technical Abstract: A general method was developed for the quantification of hydroxycinnamic acid derivatives and flavones, flavonols, and their glycosides based on the UV molar relative response factors (MRRF) of the standards. Each of these phenolic compounds contains a cinnamoyl structure and has a maximum absorbance (lamda-max) for Band I within a spectral interval of 320-370 nm. MRRF values were measured for 5 separate groups at 5 different wavelengths and then normalized between groups based on rutin. The five groups were hydroxycinnamic acids and their derivatives (326 nm), apigenins (336 nm), luteolins (348 nm), flavonol glycosides at the 3 and 7 position (354 nm), and flavonol aglycones and flavonol glycosides at other than the 3 and 7 positions (368 nm). Using MRRF values, a series of 5 standards can be used to quantify the polyphenols in each group or a single standard such as rutin can be used to quantify the phenolics in all 5 groups. The quantification of these polyphenols in two teas and Erigeron breviscapus whole plant was used as examples.