Aqueous coordination polymerization of ethylene by different neutral nickel(II) complexes [(X boolean ANDO)NiR(L)] (X = P: type 1; X = N: type 2) was investigated. With catalyst precursors of type'1, productivities and molecular weights were reduced by comparison to conventional polymerization in organic solvents. This effect results to a large extent from a lowering of the chain growth rate due to a low concentration of ethylene at the catalytically active centers in the aqueous polymerization. The catalysts are stable in water for hours. Stable latices of low-molecular-weight linear polyethylene were obtained with a hydrophilic complex of type 1 in an emulsion-type polymerization (M-w ca. 3 x 10(3) g mol(-1), M-w/M-n 2-3; solids content of polymer dispersions up to 10%). With complexes of type 2, productivities in aqueous ethylene polymerization were also reduced by comparison to polymerization in organic solvents. However, moderately branched semicrystalline polymers of high molecular weight are accessible (M-n > 10(4) g mol(-1)). Employing norbornene as a comonomer, high-molecular-weight amorphous copolymers were obtained in water as a reaction medium. Ethylene solubility in water-acetone mixtures was estimated experimentally.