This study investigated the photodebromination behaviors of polybrominated diphenyl ethers (PBDEs) in methanol or methanol/water systems. The kinetics of three sets of bromated diphenyl ether (BDE) isomers were compared in the same reactors, and the results showed that the PBDE isomers with lower energy of lowest unoccupied molecular orbital and higher energy of highest occupied molecular orbital will be degraded faster by ultraviolet (UV) light than other BDE isomers. The overall debromination pathways of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) were elucidated, and we found that the bromine substituents with higher Mulliken charges were preferentially removed. This demonstrated that Mulliken charges can be used to predict the photodebromination pathways of PBDEs. In methanol/water systems, when the ratio of methanol and water decreased, the degradation rate of BDE-47 decreased, whereas that of diphenyl ether increased. This phenomenon can be attributed to the mechanism of photodegradation of PBDEs gradually shifting from the reductive debromination to the undebromination process as the ratio of methanol and water decreases. The chromatogram of high-performance liquid chromatography and gas chromatography–mass spectrometry during this process also verified this explanation. The debromination pathways of BDE-47 are consistent in methanol/water systems with different methanol to water ratios and in different organic solvents. Display omitted •PBDEs with a narrower HOMO-LUMO gap tend to degrade faster than other BDE isomers.•Mulliken charge is a good descriptor to predict the debromination pathways of PBDEs.•The higher water content changed the degradation manner of PBDEs in methanol/water system.•The debromination pathways of BDE-47 are consistent in various solvent.