The photolytic degradation of fireMaster BP-6, a commercial polybrominated biphenyl (PBB) fire retardant, was investigated by using high-resolution, isomer-specific, fused silica capillary gas chromatography. During photolysis the more highly brominated PBB congeners diminished in concentration, while those possessing three, four, or five bromines per biphenyl increased, consistent with a reductive dehalogenation pathway. Unlike the photolysis of single cogeners, a preferential loss of ortho bromines was not found. Those PBBs which have been identified as the most toxic (i.e., those containing no ortho bromines) did, nevertheless, increase approximately 4-fold, and this increase may explain the enhanced toxicity of photolyzed fireMaster BP-6. (Copyright (c) by the American Chemical Society, 1983.)