Biodegradation of the laundry surfactant linear alkylbenzenesulfonate (LAS) involves complex bacterial communities. The known heterotrophic community has two tiers. First, all LAS congeners are oxygenated and oxidized to about 50 sulfophenylcarboxylates (SPC). Second, the SPCs are mineralized. Comamonas testosteroni KF-1 mineralizes 3-(4-sulfophenyl)butyrate (3-C4-SPC). During growth of strain KF-1 with 3-C4-SPC, two transient intermediates were detected in the culture medium. One intermediate was identified as 4-sulfoacetophenone (SAP) (4-acetylbenzenesulfonate) by nuclear magnetic resonance (NMR). The other was 4-sulfophenol (SP). This information allowed us to postulate a degradation pathway that comprises the removal of an acetyl moiety from (derivatized) 3-C4-SPC, followed by a Baeyer-Villiger monooxygenation of SAP and subsequent ester cleavage to yield SP. Inducible NADPH-dependent SAP-oxygenase was detected in crude extracts of strain KF-1. The enzyme reaction involved transient formation of 4-sulfophenol acetate (SPAc), which was completely hydrolyzed to SP and acetate. SP was subject to NADH-dependent oxygenation in crude extract, and 4-sulfocatechol (SC) was subject to oxygenolytic ring cleavage. The first complete degradative pathway for an SPC can now be depicted with 3-C4-SPC: transport, ligation to a coenzyme A (CoA) ester, and manipulation to allow abstraction of acetyl-CoA to yield SAP, Baeyer-Villiger monooxygenation to SPAc, hydrolysis of the ester to acetate and SP, monooxygenation of SP to SC, the ortho ring-cleavage pathway with desulfonation, and sulfite oxidation.