Abstract: A process for defoaming and/or degassing organic systems by addition of an organopolysiloxane-containing anti-foaming agent to the organic system is characterized by the fact that the organopolysiloxane used is one composed of siloxane units having the general formulae (I) and (II), in which R is a univalent hydrocarbon residue with 1 to 18 carbon atoms per residue; A is a residue having the general formula (a), in which R.sup.1 is a residue having the formula --CR.sup.3 H--, where R.sup.3 stands for a hydrogen atom or a univalent organic residue; R.sup.2 is a residue having the formulae --CR.sup.4 H--CH.sub.2 -- or --CH.sub.2 CH.sub.2 CH.sub.2 -- where R.sup.4 stands for a hydrogen atom or a univalent organic residue; v and w represent each O or an integer, the sum of v+w being in average 0 to 16; x and y represent each 0 or 1, the sum x+y being 1 or 2; a equals 1,2 or 3; b equals 0, 1 or 2 and c equals 1 or 2, the sum b+c not exceeding 3.