Abstract: A composition useful for defoaming and/or deaerating liquid organic systems, such as crude oil, hydraulic oil, lacquer, varnish and paint is disclosed. The composition comprises(a) a polydiene of the general formula ##STR1## wherein Z=a chain-terminating groupR.sup.1 =hydrogen or methylR.sup.2 =alkyl with 1 to 6 carbon atoms or phenylR.sup.3 =the OR.sup.5 group, in which R.sup.5 =alkyl, the (C.sub.n H.sub.2n O).sub.m R.sup.6 group, in which R.sup.6 =alkyl, acyl or alkylaryl, n=2 or 3 and m.gtoreq.1, or the C.sub.p F.sub.2p+1 (CH.sub.2).sub.2 - group in which p=4 to 16, ##STR2## R.sup.7 =alkyl with 1 to 6 carbon atoms or phenyl, q.gtoreq.3, x,y.gtoreq.0,z.gtoreq.3,a=0 to 2,b=0 to 2, anda+b.ltoreq.2, and(b) an organic solvent in which the polydiene is dissolved. A method of deformation and/or deaerating organic systems with such polydienes is also disclosed.