The experimental hypervelocity jet initiation threshold criteria for neat nitromethane and homogeneous- and heterogeneous-sensitized nitromethane mixtures have been determined over a range of failure diameters, jet velocities from 2-9 mm/mu s, and jet diameters In two test configurations. These criteria were used to define detonation and failure conditions in nitromethane and the nitromethane mixtures as a function of contact- and bow-shock conditions. These data were compared with data reported by others in the literature on bare solid explosives. A favorable comparison of threshold values is achieved by normalization of the criteria with the failure diameters. The effect on the initiation of the confinement proximity to the jet axis was also investigated. Experimental results have shown that high impedance boundaries near the jet axis induced conditions that allowed initiation to occur at lower jet velocities than those determined by the criteria. Tests also were performed on both homogeneous and heterogeneous nitromethane mixtures, having the same failure diameters, to determine if the same or similar jet penetration distances were required for initiation. The results, within experimental error, do not show any definitive differences in initiation conditions between the two types of mixtures.