Abstract: Process is disclosed for effecting phase separation of relatively high and low viscosity immiscible liquids comprising maintaining, between inner means in relative rotational movement with outer sleeve means, at least one zone of increased pressure constriction and of decreased pressure constriction; feeding a multiphase mixture of relatively low viscosity and relatively high viscosity liquids to the interior of said sleeve means between said zones of increased and decreased pressure constriction; and withdrawing said high viscosity liquid and said low viscosity liquid from the vicinities of said zones of increased and decreased pressure constriction, respectively.