A new method for the analysis of alcohol ethoxylates (AEs) using electrospray ionisation liquid chromatography/mass spectrometry (ESI LC/MS) is described. The procedure incorporates a novel derivatisation step with phthalic anhydride for the analysis of EO0–20 ethoxylates in a single analysis. The derivatives obtained have proved to be very stable and the negative ion spectra show reduced background ions and competing adduct formation as compared to positive ion spectra. An automated solid phase extraction (SPE) step is used to allow both pre-concentration and clean-up of the environmental samples. The method provides more efficient recovery of AEs across the C12–C18 range than previously reported in the literature. Recoveries from final effluent spiked at 100 ?g/L total AE, for the 126 species analysed, were found to be in the range 55–117%, with approximately 100 of the individual analytes having recoveries of 90–105%. An LOD of 0.02 ?g/L for individual ethoxylate components is reported with the instrument operated in scan mode over the range m/z 300–1300. The method was applied to sewage effluent and influent samples, with AEs determined at approximately 7 and 5000 ?g/L, respectively.