Hydrolysable tannins appear to have some extremely important biological roles including antioxidant, antibacterial, anti-inflammatory, anti-hypoglycemic, anti-angiogenic, and anticancer activities. The aim of this work was to set up a flow injection high-resolution mass spectrometric approach combined with database searching to obtain rapidly a profiling of gallotannins and other phenolics in a crude extract from plant tissue. The flow injection analysis (FIA) takes place in an electrospray ionization source of an hybrid orbitrap high resolution mass spectrometry system (ESI-HR-MS/MS(2), resolution 100,000, negative ion mode) and polyphenols are tentatively identified by matching the monoisotopic masses of the spectra with those of polyphenols databases. This leads to the most probable molecular formulas and to the possible structures among those reported in the database. The structures confirmation occurs by the compliance of MS(2) fragments with those of a prediction fragment commercial database. With this method we identified in the aqueous extract of sumac leaves, with a maximum error of 1.7 ppm, a group of ten gallotannins from mono- to deca-galloyl glycosides of the class of hydrolysable tannins and a set of coextracted flavonoid derivatives including myricetin, quercetin-3-O-rhamnoside, myricetin-3-O-glucoside, myricetin-3-O-glucuronide, and myricetin-3-O-rhamnoglucoside. The separation of isomers of gallotannins and flavonoids present in the same extract occurred by high-performance liquid chromatography with electrospray ionization tandem mass spectrometry (HPLC/ESI-HR-MS(2)); this approach allowed the structure resolution of the isobaric flavonoids quercetin-3-O-glucoside and myricetin-3-O-rhamnoside.