Polyphenol profile of Citrus juices of sweet orange, tangerine, lemon and grapefruit from Spanish cultivars was obtained by High-Performance Liquid Chromatography with Diode Array Detection coupled to Electrospray ionization and Triple Quadrupole Mass Spectrometry. Fifty eight phenolic compounds of five different classes were identified in these Citrus juices. Flavanone: O-dihexoside of naringenin; flavones: apigenin-7-O-rutinoside-4'-O-glucoside, luteolin-7-O-neohesperidoside-4'-O-glucoside, luteolin-6-C-glucoside, 6,8-di-C-acylhexosides of chrysoeriol and diosmetin, 6C- and 8C-glucoside-O-pentoside of apigenin, apigenin-6-C-hexoside-O-hexoside and apigenin-8-C-hexoside-O-acylrhamnoside; flavonols: 7-O-rutinosides of quercetin, kaempferol, isorhamnetin and tamarixetin, kaempferol-3-O-rutinoside, isorhamnetin-3-O-rutinoside-7-O-glucoside, tamarixetin-3-O-rutinoside-7-O-glucoside, isorhamnetin-3-O-hexoside-7-O-rhamnosylhexoside, 3-O-rhamnoside-7-O-rhamnosylhexoside of quercetin and isorhamnetin and kaempferol-3-O-rhamnosylhexoside-7-O-rhamnoside; hydroxycinnamic acids: O-hexoside of ferulic and sinapic acid; and, coumarins: O-hexoside and O-rhamnosylhexoside of scopoletin, had not previously been reported in Citrus juices to our knowledge. Structures have been assigned on the basis of the complementary information obtained from retention time, UV-visible spectra, scan mode MS spectra, and fragmentation patterns in MS(2) spectra obtained using different collision energies. A structure diagnosis scheme is provided for the identification of different phenolic compounds.