A major environmental concern associated with polybrominated diphenyl ethers (PBDEs) is the potential for their oxidative transformation into the toxic polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs). However, the process that PBDEs are converted into PBDFs by UV light is not well understood. Here, we report on the photolysis of ten lower PBDE congeners (≤tetra) and diphenyl ether (DE) in various solutions to explore the effect of the brominated positions of PBDEs on their transformation to PBDFs. The results suggest that the ortho-bromine substitution is the prerequisite for converting PBDEs into PBDFs in these solutions by UV light. Without an ortho-bromine substituent, PBDE congeners did not generate PBDFs, but sequentially debrominated into DE, which converted into dibenzofuran (DF). The amount of PBDFs that accumulated during the photolysis of PBDEs with one ortho-bromine substituent was much greater than the amount that from the photolysis of PBDEs that with two ortho-bromine substituents. Quantum chemistry data suggests a mechanism of the formation of PBDFs from ortho-brominated PBDEs that involves the formation of ortho-carbon radicals, followed by either an addition of a hydrogen atom extracted from a solvent molecule to form lower PBDEs, or an intramolecular reaction to form PBDFs.