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Our studies with Sphingobium xenophagum Bayram show that this bacterial strain degrades ?-quaternary 4-nonylphenols by an ipso-substitution mechanism, whereby the nonylphenol substrates are initially hydroxylated at the ipso position to form 4-hydroxy-4-nonylcyclohexa-2,5-dienones (quinols). Subsequently, the ?-quaternary side chains are able to detach as short-living cations from these intermediates. Alkyl branches attached to the carbocation help to delocalize and thereby stabilize the positive charge through inductive and hyperconjugative effects, which explains why only alkyl moieties of ?-quaternary nonylphenols are released. This view is corroborated by experiments with S. xenophagum Bayram, in which the alkyl chains of the non-?-quaternary 4-(1-methyloctyl) phenol (4-NP2) and 4-n-nonylphenol (4-NP1) were not released, so that the bacterium was unable to utilize these isomers as growth substrates. Analysis of dead end metabolites and experiments with 18O labeled H2O and O2 clearly show that in the main degradation pathway the nonyl cation derived from ?-quaternary quinols preferentially combines with a molecule of water to yield the corresponding alcohol and hydroquinone. However, the incorporation of significant amounts of O2-derived oxygen into the nonanol metabolites derived from degradation of certain ?,?-dimethyl substituted nonylphenols by strain Bayram strongly indicates the existence of a minor pathway in which the cation undergoes an alternative reaction and attacks the ipso-hydroxy group, yielding a 4-alkoxyphenol as an intermediate. Additional growth experiments with strain Bayram revealed that also the two alkoxyphenols 4-tert-butoxyphenol and 4-n-octyloxyphenol promote growth. Furthermore, strain Bayram's ipso-hydroxlating activity is able to transform also bisphenol A.