The fracture behavior of nanocomposites formed from an organoclay, based on montmorillonite (MMT), and a poly(ethylene-co-methacrylic acid) ionomer prepared by melt compounding was investigated using all instrumented impact test. The data were analyzed using the essential work of fracture (EWF) methodology. Transmission electron microscopy revealed that the clay platelets were well-exfoliated in this matrix. The fracture energy of these nanocomposites increased with organoclay addition at low concentrations but decreased with further increase in organoclay concentration with a maximum between 2 and 3 wt% MMT. The initial increase in fracture energy is a result of the higher forces during loading caused by the increase in modulus and yield stress upon addition of clay; however, the fracture energy eventually decreases with further addition of clay owing to the continuous decrease in ductility or deflection during testing. The EWF of fracture analysis showed that the energy per unit area of crack surface formed exhibits a maximum at 2-3 wt% MMT while the energy dissipated per unit volume in the surrounding process zone decreases monotonically for all clay loadings with a transition from ductile to brittle behavior occurring at 7-8 wt% MMT. (c) 2007 Elsevier Ltd. All rights reserved.