4-Alkylphenols are amphiphilic compounds commonly used as surfactants in many industrial production processes. They can have many functions, such as surfactants in a detergent or emulsifying agents in plastic or latex production. 4-Nonylphenols, an important class of 4-alkylphenols, have an estimated use of 650,000 tons annually ([http://www.ncbi.nlm.nih.gov/pubmed/11993862|Guenther et al., 2002]). This is a major concern as 4-nonylphenols are not easily degraded and their buildup can be toxic to an array of species from aquatic organisms to mammals ([http://www.ncbi.nlm.nih.gov/pubmed/9699867|Sonnenschein and Soto, 1998]). There can be as many as or more than forty isomers in technical grade 4-nonylphenol ([http://www.ncbi.nlm.nih.gov/pubmed/16427065|Moeder et al., 2006]). Microbes have evolved to biodegrade some of the relatively resilient 4-nonylphenol isomers. The process appears to depend on molecular oxygen ([http://www.ncbi.nlm.nih.gov/pubmed/15091914|Ekelund et al., 1993]), suggesting activation of the compound by incorporation of oxygen-bearing functional groups. To better understand 4-nonylphenol catabolism, one particular isomer, 4-(1-ethyl-1,4-dimethyl-pentylphenol) or NPL, is shown here in detail, with the R stereoisomer as the initial substrate. Both enantiomers are degraded, but not much is currently known regarding stereoselectivity. Sphingomonas spp. carry out the initial biodegradation of NPL. A major and a minor degradation pathway have been postulated, both ultimately leading to hydroquinone and a nonanol that retains the structure of the original nonyl side chain. Sphingomonas spp. can grow on the aromatic part as a carbon and energy source, but are not capable of further catabolizing the nonyl alcohol, and it is discarded as a non-catabolic byproduct ([http://www.ncbi.nlm.nih.gov/pubmed/15665329|Gabriel et al., 2005]). Both the minor and major pathways are catalyzed initially by an ipso-hydroxylation, i.e., hydroxylation at the anchor carbon atom of the alkyl substituent, forming 4-nonyl-4-hydroxy-cyclohexadienone ([http://www.ncbi.nlm.nih.gov/pubmed/15665329|Gabriel et al., 2005]). 18O-labeling experiments show that the ipso-hydroxy group is derived from molecular oxygen and that the major pathway for cleavage of the alkyl moiety is through cleaving the nonyl chain as alkyl cation (stabilized by the α-quaternary C atom of the nonyl group) and combination of the cation with a molecule of water to yield the nonyl alcohol and hydroquinone. In the minor pathway (not shown), a 1,2-C,O shift of the alkyl moiety is postulated to yield a 4-alkoxyphenol intermediate. A second ipso-hydroxylation leads to a hemiketal, which then spontaneously decomposes to p-benzoquinone and the nonyl alcohol. p-Quinone is quickly converted to hydroquinone by a reducing system ([http://www.ncbi.nlm.nih.gov/pubmed/17369338|Gabriel et al., 2007]).