A rapid compression machine is used to study the autoignition characteristics of JP-8 and JP-5 blended in 50/50 mixtures by volume with five proposed alternatives, two Fischer-Tropsch fuels and three solvents. Several implicit properties of alternative fuels related to their physical and chemical properties prompted a study of the influence of chemical composition on autoignition, including the influence of isoparaffinic, cycloparaffinic, and aromatic structures. The tests employ the direct test chamber charge preparation method which allows for high reproducibility of measurements. Ignition delay measurements are compared between the conventional military jet fuels, neat alternatives, and the 50/50 blends. Measurements are made at a compressed pressure of 20 bar at equivalence ratios of 1.0 and 0.5 and temperatures between 630 K and 720 K.