The invention provides surface-active random oligomers of the general formula:R.sub.1 --O--C.sub.2 H.sub.3 (Z)--O].sub.n H (I)in which R.sub.1 denotes a linear or branched, saturated or unsaturated aliphatic hydrocarbon radical or a cyclo-aliphatic, aromatic or alkylaromatic radical containing 1 to 30 carbon atoms, Z denotes a group A and/or at least one group corresponding to the formula (II)R.sub.2 --O--C.sub.2 H.sub.3 (A)--O].sub.m CH.sub.2 -- (II)in which R.sub.2 denotes a hydrocarbon radical having the meaning indicated for R.sub.1 and containing a number of carbon atoms from 8 to 30, n denotes an average statistical value from 1 to 20, m denotes an average statistical value from 0.5 to 10 and A denotes a group of hydrophilic character. These oligomers can be used in cosmetic or pharmaceutical compositions.