Bis(phenoxy-imine) Zr complexes upon activation with Et-3/Al(Ph3C)(m)H-n[PMo12O40].8 H2O (average: m/n = 2:1) were demonstrated to be highly active catalysts for the polymerization of ethylene. One of the complexes formed narrow-molecular-weight distributed polyethylene ((M) over bar (w)/(M) over bar (n) 1.45) with a very high activity (5640 kg-PE . mol-cat(-1) .h(-1)), representing the first example of a MAO- and borate-free, highly active, single-site catalyst system based on a Group 4 transition metal complex and a heteropoly compound.