Catalytic polymerization of ethylene in aqueous emulsion by in situ catalysts based entirely on compounds commercially available from standard chemicals suppliers, namely a chlorinated derivative of 1,4-benzoquinone, triphenylphosphine, and bis(1,5-cyclooctadiene)nickel, is reported. Using miniemulsions of catalyst solutions in a small amount of hydrocarbon solvent for polymerization, stable dispersions of linear semicrystalline polyethylene are obtained. With 2,3,5,6-tetraebloro-1,4-benzoquinone as the quinone component, polyethylenes with molecular weights (M-n up to 3 x 10(3) g mol(-1)) clearly exceeding the entanglement limit are accessible. The catalyst is stable in water for hours with productivities of up to 2 x 10(3) mol(ethylene) mol(Ni)(-1) h(-1). Productivities and molecular weights are in the same range for conventional polymerization in organic solvents. Polyethylene dispersions with solids contents of up to 28% are obtained. Aqueous dispersions of ethylene/1-butene copolymers with comonomer incorporations of up to 3 mol % (6 wt %) were prepared.