Two metabolites of branched alkylbenzene sulfonates, 5-methyl-6-(4-sulfophenyl)heptanoic acid and 3,3,4-trimethyl-5-(4-sulfophenyl)hex-4-enoic acid were tentatively identified respectively in 8 and 10 out of 10 samples. MS/MS spectra showed a high number of characteristic product ions, highly informative about their alkyl chains structures (details in Table ). Despite this richness of MS/MS information and the high probability of paraposition substitution, no certain structure based solely on MS data could be assigned and they were reported at Level 3.