The fracture behavior of elastomers has been studied in the high speed region immediately above a transition region where a change in the nature of the fracture surface occurs. A video camera operating at up to 6000 frames/s was used to follow the progress of failure. The test pieces employed were of the pure shear or tensile strip (with edge crack) varieties employed in earlier tear studies; in order to attain the high energy release rates required they were usually pre-strained and then cut to initiate the crack growth. The materials studied included various strain crystallizing and non-crystallizing elastomers, with different glass-transition temperatures, including natural rubber in normal, cis—trans isomerized or partially-epoxidized forms, a butadiene—acrylonitrile copolymer, a styrene—butadiene copolymer and ethylene—propylene co- and ter-polymers. The use of a fracture mechanics approach based on the strain energy release rate enables results for different test piece geometries to be brought into agreement in the region above the transition. Fracture energies in this region correlate well with viscoelastic properties, but the potential for strain crystallization to strengthen is not exhibited. This is presumably because at the high failure rates involved, the loading time at the crack tip is insufficient to allow significant crystallization to occur. The correlation with viscoelastic behavior suggests that the material around the propagating tip is still essentially rubbery in its behavior, although the fracture surfaces in the high speed region have a smooth, “glassy” appearance. Further evidence of this is provided by effects of crosslink type and density and filler type and loading, where again the effects seen in the high speed failure region parallel those observed below the transition. An effect of thickness on fracture properties appears to be absent in the high speed region, by contrast with behavior at lower energies and at the transition itself. This supports the view that the fracture surface roughness that develops in the lower energy region is due to the initiation of a process akin to cavitation by through-the-thickness stresses near the crack tip. The transition is found to vary with pressure and temperature, as well as thickness, in a way that does not correlate with viscoelastic changes but may reflect changes in the through-the-thickness stresses. The existence of a limiting crack speed is illustrated and discussed. Fracture under large biaxial deformations, where higher crack speeds are observed, will be discussed in Part II.