A polymerization process comprising mixing siloxane polymers with organosilicon compounds having at least one silicon-bonded group RN, which is a substituent comprising at least one amine group, and with a phosphazene base catalyst and allowing the siloxanes and organosilicon compounds to polymerize to form amino-functional polyorganosiloxane polymers is claimed. Preferably the phosphazene base is selected from the following formulae: ((R12N)3P═N—)x(R12N)3−xP═NR2; {((R12N)3P═N—)x(R12N)3−xP—N(H)R2}+{A−} {((R12N)3P═N—)y(R12N)4−yP}+{A}− or {(R12N)3P═N—(P(NR12)2═N)n—P+(NR12)3}{A−} in which R1 is hydrogen or an optionally substituted hydrocarbon group, or in which two R1 groups bonded to the same N atom may be linked to complete a heterocyclic ring, R2 is hydrogen or an optionally substituted hydrocarbon group, x is 1, 2 or 3; y is 1, 2, 3 or 4, n is an integer with a value of from 1 to 10 and A is an anion.