The toughening mechanisms in rubber-modified epoxies appear to be viscoelastic in nature since their fracture behavior is dependent on loading rate. This behavior has been studied in detail and modeled for only one system, a model toughened epoxy often used in research work. The present study examines the loading rate effect for a new material based on acrylic rubber by measuring the fracture energy in constant cross-head speed tests conducted over a wide range of speeds. As expected, decreasing the loading rate produced an increase in toughness. Just as in the previous studies, the fracture energies could be modeled with a power law relationship when the loading rate was characterized by the time of failure. Moreover, the parameters involved in the model are quite consistent with the earlier results. For most rates, the behavior was approximately linear elastic with little or no r-curve behavior. Below a critical rate, however, there was a transition to ductile failure with a large r-curve and very high fracture energies. The transition is very sudden which may help explain why some previous studies have observed this effect while others have not.