Abstract: The invention relates to a organo-tin-containing complex stabilizer for synthetic resin compositions of the formula I:M.sup.2+.sub.1-a-b(2+x) Al.sup.3+.sub.a [Sn(R).sub.(2-x) ].sub.b.sup.(2+x)+ (OH).sup.-.sub.2-b(2+x) A.sup.n-.sub.a/n *mH.sub.2 O (I)wherein M.sup.2+ stands for at least one divalent metal cation, R stands for a C.sub.1-12 linear or branched alkyl group, A.sup.n- stands for an n-valent anion or mixtures of anions, and the following conditions apply:0<a<0.5; 0<b<0.1; 0.ltoreq.x.ltoreq.1; 0<a+b(2+x)<0.5; 0.ltoreq.m.ltoreq.2.The invention also relates to a method for the preparation of these stabilizers and their use in synthetic resin compositions, preferably halogen-containing synthetic resin compositions, or (co)polymers obtained by Ziegler Natta polymerization processes.