The anaerobic microbial and photochemical degradation pathways of 4,4'-dibromodiphenyl ether (BDE15) were examined. BDE15 was reductively debrominated within a fixed-film plug-flow biological reactor at hydraulic retention times of 3.4 and 6.8 h, leading to exclusive production of 4-bromodiphenyl ether (BDE3) and diphenyl ether (DE). A suite of potential BDE15 metabolites arising from reductive debromination, hydroxylation, and methoxylation of the aromatic C-Br and C-H bonds were not observed. Following initial debromination of BDE15, degradation of BDE3 to DE readily occurs, suggesting the rate-limiting step for anaerobic BDE15 degradation is conversion of BDE15 to BDE3. The photochemical degradation of BDE15 was also examined in organic (CH3CN and CH3OH) and aqueous (H2O:CH3CN; 1:1 v/v) solvent systems at 300 nm. Only photochemically induced reductive debromination was found to occur via homolytic C-Br bond cleavage, with no evidence of C-O bond cleavage or products arising from heterolytic bond cleavage.