Homoleptic and mixed a-zirconium phosphonates (ZrPs) a-Zr(O3PR)2 (R = Me, Bun, Bui, Hex, Oct and Ph) and a-Zr(O3PR1)2-x(O3PR2)x were readily prepared in high yields from zirconyl choride and the corresponding phosphonic acids in suitable solvent mixtures under hydrothermal conditions at low fluoride concentrations. They form crystalline aggregates consisting of platelets from ca. 10–20 monolayers thickness, with well-defined surface structures. Impregnation with Cp2ZrCl2 by sublimation or slurry methods provided the first examples of ZrP-supported alkene polymerization catalysts. Crystal morphology and interlayer spacing are unaffected by the impregnation process. Solid-state NMR spectroscopy provides evidence for the integrity of the adsorbed metallocene structure. Covalent attachment of Cp*ZrCl3 to functionalized ZrPs of the type a-Zr(O3PR1)1.8(O3PCnH2nOH)0.2 is similarly possible. The new catalysts polymerize ethene with good to excellent activities under mild conditions, even at remarkably low methylalumoxane/zirconocene ratios of 10:1. The polymer is obtained as free-flowing particles, which reflect the morphology of the catalyst supports.