Pseudomonas strains K22 and KF46 had previously been isolated from chemostat cultures that were adapted to growth on 1-(4'-carboxyphenylazo)-4-naphthol (carboxy-Orange I) and 1-(4'-carboxyphenylazo)-2-naphthol (carboxy-Orange II), respectively. They were tested for their ability to degrade the sulfonated analogs 1-(4'-sulfophenylazo)-4naphthol (Orange I) and l-(4'-sulfophenylazo)-2-naphthol (Orange II). The sulfonated dyes served as models for commercially used textile dyes, which are known to be recalcitrant in aerobic waste water treatment plants. Substitution of sulfo for carboxy groups led to disturbance of the degradative pathways. The enzymes initiating degradation, the Orange I azoreductase and the Orange II azoreductase, accepted both, carboxylated and sulfonated dyes. Experiments with specifically 14C-labelled dyes indicated that sulfanilic acid, one of the products of the initial fission of the dyes, was channeled into a dead-end pathway. In the case of Orange I degradation, reactive metabolites of sulfanilic acid, presumably catechols, coupled with aminonaphthol, the other product of the azoreductase reaction. Orange II was degraded by strain KF46 when another suitable carbon source (e.g. 4-hydroxybenzoate) was supplied. Most but not all of the internally generated sulfanilic acid was excreted and intermolecular coupling of aromatic metabolites was not observed. However, the presence of sulfanilic acid and/or its metabolities still interfered with the degradation of the aminonaphthol part of the dye molecule and complete mineralization was not achieved.