The effect of impact modification on slow crack growth in a poly(vinyl chloride) (PVC) compound was examined in order to test a methodology for predicting long-term creep fracture from short-term tension-tension fatigue tests. In all cases the crack propagated in a stepwise manner through a crack tip craze zone. Step length was analyzed in terms of the Dugdale model for a crack tip plastic zone. The overall crack growth rate in fatigue and creep followed the conventional Paris power law with the same power 2.7, da/dt = A(f) DeltaK(1)(2.7) and da/dt = BK12.7, respectively. The effects of frequency, temperature, and R-ratio ( the ratio of the minimum to maximum stress intensity factor in the fatigue loading cycle) on the Paris prefactor were determined. Crack growth rate was modeled as the product of a creep contribution that depended only on the maximum stress intensity factor and a fatigue contribution that depended on strain rate da/dt = BfKl,max2.7 (1 + C(epsilon) over dot), where C is a coefficient defining the strain rate sensitivity. A linear correlation allowed for extrapolation of the creep prefactor B-f from fatigue data. Impact modification decreased B-f but had no effect on C. (C) 2004 Kluwer Academic Publishers.