In this study. we compared the lipophilicity of O-glucuronides and their aglycones. Distribution coefficients (log D) and P values of neutral species (log P) were determined by centrifugal partition chromatography (CPC) in octanol:buffer systems. Two-phase potentiometry was also used to measure the log P value of some lipophilic solutes. The experimentally determined global influence of glucuronidation on lipophilicity. obtained as the difference (decrement) log P-(glucuranide) - log P-(aglycone), was found to be - 1.30 +/- 0.16 (n = 4) for glucuronides of alcohols (methyl. menthyl, neomenthyl, and chloramphenicol O-glucuronide). The mean decrement was - 2.06 +/- 0.31 (n = 9) for glucuronides of phenols (phenyl, p-nitrophenyl, 1-naphthyl, 6-bromo-2-naphthyl, 4-methylumbelliferyl, 3-coumarinyl, phenolphthalein, 4-benzophenonyl O-glucuronide, and diflunisal phenolic glucuronide). For the acylglucuronide of diflunisal and its rearrangement isomers, the mean decrement was - 1.80 +/- 0.08 (n = 4; range - 1.7 to - 1.9). Differences in through-bond proximity effects as parametrized in the CLOGP algorithm seem to account for much of this difference. Conformational factors may also play a role, although it appears modest and unassessable for the glucuronides investigated here. The results imply that in vivo glucuronidation should have a stronger influence on the excretion of phenols than on that of alcohols