The invention relates to a catalyst for preparing organosiloxanes and polyorganosiloxanes which are obtained primarily by polycondensation processes. The catalyst of the invention is a reaction product of phosphonitrilic chloride with a compound of the general formula �R.sub.3 SiO(R.sub.2 SiO).sub.m !.sub.3 P.dbd.O (I), where R are, independently of one another, identical or different, unsaturated and/or saturated monovalent hydrocarbon radicals having from 1 to 6 carbon atoms or hydrogen, with the proviso that only one hydrogen atom is bound to each silicon atom and m has a value between 0 and 1000. Volatile chlorine-containing silicon compounds formed during the reaction can, if required, be completely or partially removed.