A detailed study of the fracture behavior of polyarylsulfone was conducted over a temperature range of −175 to 120°C. Both fatigue crack propagation and fracture toughness tests were run as well as forced torsion pendulum tests to characterize the dynamic properties. The polymer exhibited a broad secondary loss peak and a high glass transition temperature at −110 and 295°C, respectively. Fracture toughness, KIC, and fatigue crack growth resistance were found to vary similarly with temperature, minima being observed near −50°C. Below that temperature, both a rise in toughness and in fatigue resistance is associated with the broad secondary loss peak. The slopes of the log fatigue crack velocity (da/dN) vs log stress intensity range (ΔK) curves varied from 2.6 to 13.2. Since the equation da/dN = α(ΔK)n described all of the data, the log‐log slope or exponent, n = ∂ln(da/dN)∂ln ΔK, was considered as a stress intensity sensitivity index with respect to fatigue behavior. This index was at a maximum near −50°C, where the minimum in toughness occurred. A kinetic model was utilized to correlate the stress intensity sensitivity index and suggested that a single thermally activated mechanism controls the low temperature mechanical behavior of polyarylsulfone.