AbstractBoth liquid and solid surface‐active adducts of brassylic (tridecanedioic) acid were prepared by potassium hydroxide‐catalyzed addition of ethylene oxide gas to the molten acid. The number‐average molecular weights of the adducts ranged from 500 to 3,000. These adducts cover a wide span in the ratios of hydrophilic to lipophilic portions of the molecule. They are unique in that hydrophilic end‐groups sandwich a rather long chain of lipophilic methylene units. Treatment of some adducts with hot, saturated, aqueous sodium chloride induced transesterification and produced hydroxyl‐terminated, multi‐brassylic, poly(ethylene glycol) esters of MWn 1,000 to 4,000, all of which were efficient surface‐active agents.Average degrees of polymerization of the poly(ethylene oxide) (PEO) chains were determined by a) functional group analysis, b) weight increase, and c) methanolysis of the products, followed by gas‐chromatographic analysis of the isolated poly(ethylene glycol) fraction. Surface tension, critical micelle concentration, emulsion stability, and efficiency as an emulsifying agent in the aqueous polymerization of vinyl acetate were measured to evaluate surfactant properties of products in which the degrees of polymerization of the PEO chains ranged from 1.4 to 44.