Mixtures of catenate and cyclic siloxane oligomers of formula I and II ##STR1## wherein the substituents R consist of 3-methacryloxypropyl or 3-acryloxypropyl groups and methoxy, ethoxy and/or propoxy groups alone or together with C.sub.1 -C.sub.18 alkyl, fluoroalkyl, isoalkyl or cycloalkyl groups and/or C.sub.6 -C.sub.12 aryl groups and where not more than one 3-methacryloxypropyl or 3-acryloxypropyl group is attached to one silicon atom and the degree of oligomerization of compounds of the formula I is within the range 2