Sea buckthorn (Hippophae spp) has been used extensively in folk medicine of several Asian and European countries for hundreds of years. Being an excellent source of unsaturated fatty acids, vitamins C and E, carotenoids, phytosterols and flavonoids, it has become the most sought after medicinal shrub in Western Canada. The Hippophae rhamnoides ssp. mongolica, introduced to the Canadian prairies from Siberia in the 1930s was used as a parent material for the development of a cultivar named 'Indian Summer"'. In contrast to sea buckthorn berries and seeds, very limited studies on natural compounds, notably flavonoids, from leaves have been conducted to date. Our earlier research has revealed that the leaves are a slightly richer source of flavonoids than the berries, and that 'Indian-Summer' biotype R-C4 contained consistently the highest amount of flavonoids (1.8 to 2.0%). Flavonoids were separated from crude 30% methanol extract of dry R-C4 leaves using a RP-C18 chromatographic cartridge, end then further fractionated by LP-LC on a LiChroprep RP-18 column using gradient elution. Two pure flavonol glycosides with quercetin as the aglycone were isolated, namely quercetin 3,7-O-diglucoside (MW 626), quercetine 3-O-glucoside-7-O-rhamnoside (MW 610). Quercetin 3-O-glucoside- 7-O-rhamnoside is the dominant flavonol in the leaves and account for ca. 24% flavonoids, followed by quercetin 3.7 -O-diglucoside ca. 14% flavonoids. The chemical structure of two quercetin glycosides were determined using negative-ion FAB-MS, 1H-NMR, 13C-NMR, UV spectra, bathochromic shifts after addition of a diagnostic reagent, and by comparison of our data with literature information. Presence of quercetin as the aglicone was also confirmed by acid hydrolysis of isolated compounds followed by HPLC analysis of hydrolysis products using quercetin as a reference standard.