Abstract: Viscous liquids containing polymeric materials which will interact with one another upon contact and generate gas for foam expansion are intimately and homogeneously mixed in a very short period of time by passing them successively and continuously through first and second rotary dynamic shearing mixers, the first mixer being considerably smaller than the second mixer, and the mixture experiencing a brief period of respite from shearing between the two mixers. The mixture experiences a much higher degree of shear in the first mixer, where the heat necessary for promoting the reaction is provided through the shearing action but without creating localized hot-spots. The process is particularly well suited for the preparation of foamed polydiorganosiloxanes, on a continuous, commercial scale.