In this paper, the absorption and fluorescence spectra of xanthene, 9-trimethylsilyl xanthene (1) and 9-methyl-9-trimethylsilyl xanthene (2) are presented and discussed with the help of CNDO/S and ab initio calculations. Introduction of the trimethylsilyl group at the 9-position of the xanthene molecule considerably shifts the short-wavelength band (246nm) to the red and reduces the fluorescence quantum yields (Φ fl =0.03 for xanthene, 0.008 for 1 and <10 −4 for 2 in MeCN) and the fluorescence lifetimes (τ fl =7.4ns for xanthene, 220ps for 1 and <100ps for 2 in MeCN), while it does not affect seriously the long-wavelength band and the singlet excited state energies (E S =97.7kcal/mol for xanthene, 94.5kcal/mol for 1 and 97.4kcal/mol for 2 in MeCN). Ab initio calculations predict a ‘roof-like’ structure for 1 with folding angles 30° for the S 0 and 20° for the S 1 state. Laser (248, 266 and 308nm) and lamp photolysis (254nm) of 1 and 2 (MeCN or cyclohexane) results in [1,3]-trimethylsilyl rearrangement into the ortho-position of the xanthene moiety in the sense of a photo-Fries type reaction. The corresponding photo-Fries intermediates (exocyclic cyclohexatrienes: trimethyl-(1H-xanthen-1-yl)-silane, 1CHT and trimethyl-(9-methyl-1H-xanthen-1-yl)-silane, 2CHT) are formed within the 20ns laser pulse and show absorption spectra peaking up at 410 and 403nm, respectively. Additionally, small amounts of the corresponding 9-xanthyl radicals were detected as a result of the CSi bond rupture. Using ps-laser flash photolysis (266nm laser, MeCN) we observed a broad absorption spectrum peaking up at 960nm and decaying monoexponentially with a lifetime of 130ps, close to the measured fluorescence lifetime. We assigned therefore this transient to the singlet excited state of 1 (S 1 →S n absorption). We assume the S 1 state as the origin of the photo-Fries rearrangement, giving via CSi bond dissociation a singlet geminal radical pair (9-xanthyl radical+SiMe 3 ). In the next step, the radical pair undergoes predominantly in-cage recombination to the persistent photo-Fries intermediates with high quantum yields (Φ 1CHT =0.70 and Φ 2CHT =0.50), while to a lesser extent it escapes the solvent cage (Φ=0.30 for 1) and undergoes typical free radical reactions (e.g., scavenging with O 2 ). The estimation of the above quantum yields was possible only after determination of the absorption coefficients (ɛ) of the photo-Fries intermediates [ɛ 1CHT (410nm)=16,800M −1 cm −1 and ɛ 2CHT (403nm)=15,900M −1 cm −1 ] using three independent methods; this represents the first example in the literature.