New ethoxylated monofatty acid esters are obtained by esterification of 10-undecenoic acid chloride with several well defined polyethylene glycol mono-Me ethers. The monomeric surfactants show interesting surface properties, and they are characterized and compared to other known ethoxylated alcs. (Brijs) and acids (Mirjs). The crit. micelle concn. and area per mol. at the air-water interface are much higher than the similar com. Brijs, due to the special structure of the new surfactant allowing preferred orientation at the interface. [on SciFinder(R)]