The organosilicone surfactant Silwet L-77 (L-77), used as an agrochemical adjuvant, is a mixture comprised predominantly of [(CH 3 ) 3 SiO] 2 -(CH 3 )Si-(CH 2 ) 3 -(OCH 2 CH 2 ) n -OCH 3 oligomers (n = 3-16, average n ≈ 7.5). The commercially available L-77 mixture was purified by reversedphase high-performance liquid chromatography (HPLC) to obtain individual trisiloxane surfactant components. Pure oligomers (n = 3, 6 and 9) were also synthesized. Synthesis was achieved by hydrosilylation of monomeric ethoxylate monomethyl ether starting reagents. Pure hexa-and nonaethylene glycols were produced by condensation of smaller oligomers.Atmospheric-pressure ionization mass spectrometry (MS) methods were used to characterize fully the commercial L-77 product and synthesized or isolated components. The application of Fouriertransform ion cyclotron resonance MS and online HPLC-electrospray ionization MS techniques to the analysis of this surfactant are described here. The application of these analytical techniques also enabled elucidation of the synthetic by-products present in the commercial formulation.In addition, physico-chemical properties specific to agrochemical uses, such as droplet spread areas on plant foliage and surface tension for the different oligomer solutions, are also reported.