The specification describes preparation of materials comprising selected boron anions and processes using them as catalysts for condensation reaction of compounds having silicon-bonded hydroxy or alkoxy groups. Preferred anion compounds comprise borates incorporating quadri-substituted boron in which the substituents include highly halogenated hydrocarbon groups for example pentafluorinated phenyl groups or bis (trifluoromethyl) phenyl groups. Preferred anions are {B(C6F5)4}− and {B(C6(CF3)2H3)4}−. The specification describes condensation in presence of these materials of compounds having at least one unit according to the general formula RoaR1bR2cSiO(4−(a+b+c)/2) in which each Ro is the same or different and represents a hydroxy, alkoxy, alkoxyalkoxy or hydrocarbonoxy group having up to 10 carbon atoms, each R1 is the same or different and represents hydrogen or a monovalent substituted or unsubstituted hydrocarbon group, each R2 is the same or different and represents a divalent substituted or unsubstituted alkylene or oxyalkylene group, a has a value of 1, 2, 3 or 4, b has a value of 0, 1, 2 or 3 and c has a value of 0, 1, 2 or 3.