Naturally occurring phenols were evaluated as mediators for the transformation of the fungicide cyprodinil by birnessite. With birnessite alone, cyprodinil transformation was negligible (0.6%). In the presence of mediators, however, it increased considerably (1.5-60.9%). With some exceptions (2,6-dimethoxyphenol, syringic acid), methoxylated phenols showed a substantial capacity for enhancing the transformation. Mass spectrometry indicated that cyprodinil cross-coupled with free radicals of phenols formed at birnessite surfaces. The extent of cyprodinil transformation in the presence of syringaldehyde, m-methoxyphenol, or vanillin increased with the amount of birnessite. In reactions with o- and p-methoxyphenol and vanillic acid, cyprodinil transformation was unaffected by the amount of birnessite, but it increased with increasing phenol concentration. The addition of various humic acids at low concentrations (5-10 mg/L) had little effect on cyprodinil transformation in the presence of o-methoxyphenol or syringaldehyde. At higher concentrations, however, humic acids inhibited the transformation (by 5-20%) when o-methoxyphenol was a mediator, but showed no effect in the presence of syringaldehyde. (C) 2004 Elsevier Ltd. All rights reserved.