Mixed-surface octyl/methoxyundecyl alpha-zirconium phosphonates (ZrPs) were investigated as distributed nanoscale fillers, in concentrations up to 50%, w/w, for the purpose of increasing the elastic modulus and yield strength of polycaprolactone (PCL) without a meaningful reduction of its ductility. The volumetric nanoparticle loadings were estimated to be over 70% higher than those in nanocomposites with comparable weight fractions of nanoclay. The mechanical properties of the ZrP/PCL nanocomposite were evaluated with tensile, flexural, and dynamic mechanical testing methods. Nanocomposites containing 5% w/w ZrP showed significant increases in both the tensile yield stress and elastic modulus without any loss of ductility versus the unfilled polymer. Layer delamination from the ZrP tactoids was minimal. Kinetic barriers and the strong interlayer attraction between the ZrP surfaces limited intercalative penetration of the ZrP tactoids. ZrP loadings of 20% w/w or more resulted in the agglomeration of tactoids, leading to defect structures with a loss of strength and, at the highest loading, ductility. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 993-1001, 2009