Irradiation of 6,7-benzobicyclo[3.2.0]hepta-3,6-dien-2-one (2) in a rigid glass at -196°C leads to the formation of 3,4-benzotropone (1), which exhibits a characteristic UV-vis absorption extending to 535 nm and dimerizes to give syn-and anti-[π8 + π10] dimers 7 and 8 upon thawing the glass. The rate constant of the thermal dimerization of 1 is 12 ( 3 M -1 s -1 at -78°C. Photochemically, 1 is stable under matrix isolation, but undergoes [π10 + π10] dimerization in solution. Compound 1 in a CFCl 3 -CF 2 BrCF 2 Br-CHCl 3 glass at -155°C shows an IR band at 1506 cm -1 which is assigned to a vibration primarily involving CdO stretching mode by comparing the spectrum with that of 18 O-labeled 1. Under the same conditions tropone exhibits a CdO vibrational band at a significantly higher frequency (1553 cm -1 ), suggesting that the 3,4-benzo annulation makes the tropone ring more strongly polarized. The large contribution of a polar resonance form 1b is reflected in the high affinity of 1 for protonation: 1 is nearly half-protonated in a 0.1 M solution of CHCl 2 CO 2 H in EPA (a 5:5:2 mixture of ether, isopentane, and ethanol). Furthermore, the electronic absorption spectrum undergoes a characteristic blue shift when the medium is changed from hydrocarbons to alcohols, suggesting that 1 is more polarized in the ground state than in the excited state. The generation of 1 from 2 is thermally effected also and, upon heating a mixture of 2 and olefin such as maleic anhydride or ethyl vinyl ether in benzene at 220°C, adducts of the latter to 1 are obtained. The photochemical behavior of 2 is dependent on the state of medium in which 2 is irradiated; the photolysis of 2 in rigid glass results almost exclusively in the valence isomerization providing 1, while in fluid solution predominantly in the di-π-methane rearrangement affording 3,4-benzotricyclo[3.2.0.0 2,7 ]hept-3-en-6-one (19). Plausible causes of the medium-dependent photochemical behavior of 2 are discussed.