This study found that MCB photolysis, attacked by OH radicals, yielded 2-chlorophenol (2CP), 4-chlorophenol (4CP), 3-chlorocatechol (3-CC), chlorohydroquinone (CHQ), and 4-chlorocatechol (4-CC) as the aromatic intermediates. Oxalic acid was also found as ring opened intermediate. Experimental results showed that hydroxylation occurred prior to the dechlorination preceding ring cleavage. The illumination times for complete dechlorination and ring cleavage of the MCB were 20 minutes and 40 minutes, respectively. After ring cleavage, other reactions were needed to mineralize the organic acids into carbon dioxide and water. (C) 1998 Elsevier Science Ltd.