Decabromodiphenyl ether (BDE209) is a widely used flame retardant, yet information regarding its environmental transformation rates and pathways are largely unknown. Because photochemical transformation is often suggested as a potentially important fate process for BDE209, the reaction rate and products of the solar degradation under favorable solvent conditions were determined in this study. Decabromodiphenyl ether (BDE209), dissolved in hexane, reacts in minutes via direct solar irradiation, at midlatitude (40 degrees 29' N, 86 degrees 59.5' W) in afternoon July and October sunlight. Observed first-order reaction rate constants, kobs, at the different exposure times were kobs = 1.86 x 10(-3) s(-1) (July) and kobs = 1.11 x 10(-3) s(-1) (October). The photodecomposition quantum yield was calculated from these data and from the solar irradiance data measured at 300, 305.5, 311.4, 317.6, 325.4, 332.4, and 368 nm reported at a USGS UVB monitoring station located nearby. The range of wavelengths where both the molar absorptivity of BDE209 and the solar irradiance flux are significant occurs between 300 and 350 nm. For this range, the wavelength average quantum yield for BDE209 photoreaction, phiave, was calculated to be 0.47. The difference between kobs values at the two exposure times is explained fully by the difference between the solar irradiation fluxes. Upon solar irradiation, BDE209 reductively dehalogenated to other polybrominated diphenyl ethers (PBDEs). During 34 h of irradiation, PBDEs ranging from nona- to tri-bromodiphenyl ethers were observed. In total, 43 PBDEs were detected, and the GC retention times and mass spectral fragment patterns of 21 products matched those of available congener standards, including congeners 2,2',4,4',5-pentabromodiphenyl ether and 2,2',4,4'-tetrabromodiphenyl ether.