Abstract: This invention relates to a method of surface treating particulate inorganic fillers with a surface-treating agent which is an equilibrated reaction mixture consisting essentially of R.sub.2 SiO.sub.2/2 units, RSiO.sub.3/2 units and CH.sub.3 O.sub.1/2 radicals wherein the mixture contains A moles of R.sub.2 SiO.sub.2/2 units per mole of RSiO.sub.3/2 units and B moles of CH.sub.3 O.sub.1/2 radicals, A having an average value of from 0.5 to 10 and B having an average value of from 3 to 6, the value of B being related to A such that when A=0.5, B=6 and when A=10, B=3, and R being a hydrocarbon radical of from 1 to 10 inclusive carbon atoms or a haloalkyl radical of from 1 to 10 inclusive carbon atoms.