Electron spin resonance (ESR) spectrometry was used to study Cu(II) complexes with anionic surfactant compounds of the types that have been detected in sewage sludge or in the fulvic acid fraction of anaerobically-digested sewage sludge and found to resist biodegradation in the soil environment. The ESR spectra of frozen (77 ยฐK), aqueous solutions of linear alkyl benzene sulfonates and fatty alcohol sulfate esters at Cu-ligand molar ratios ranging from 0.1 to 1 exhibited anisotropic patterns indicative of a dx2 _y2 groundstate of Cu(II) bound into innersphere complexes with the ligands arranged in square planar coordination. Sulfonate-type surfactants, both in the acid and salt forms, appeared to complex Cu(II) more efficiently than ester sulfate-type surfactants. Conventional physical parameters calculated from the ESR spectra were consistent with fully oxygenated, 4 O-ligand binding sites for the Cu(II) ions and indicated the formation of similar unidentate or bidentate complexes between Cu(II) and the surfactants at any Cu/ligand molar ratio investigated. The spectral lineshapes and related parameters of the Cu(II)-anionic surfactant complexes, however, were not very similar to those observed previously for Cu(II)-sewage sludge FA complexes. It was concluded that anionic surfactants involved in Cu(II) complexation by sewage sludge or sewage sludge fulvic acid do not behave as isolated, independent ligands, but instead may participate as co-ligands with other O-containing functional groups and/or as moities incorporated into the fulvie acid structure.