Conventional and synthetic alternative jet fuels possess distinctive chemical compositions and physical/chemical properties since they are produced through different production processes. In this paper, the autoignition characteristics of four conventional aviation jet fuels (Jet-A POSF-4658, Jet-A POSF-10325, JP-5 POSF-10289, JP-8 POSE-6169), five alternative jet fuels (Syntroleum S-8 (S8), Shell synthetic paraffinic kerosene (Shell SPK), Sasol iso-paraffinic kerosene (Sasol IPK), hydro-processed renewable jet (HRJ8), alcohol to jet (ATJ), and 50/50 vol% blends of JP-8/alternative jet fuels were explored. The objective of the current work is to understand how the chemical composition of the fuels affects the gas phase chemical kinetic behavior and the ignition process of the liquid spray in an engine-relevant environment, thereby ultimately enabling their use in military ground vehicles with compression ignition engines. A modified Cooperative Fuel Research (CFR) motored engine was utilized and the gas phase ignition behavior was observed over a wide range of temperatures and pressures, while varying the compression ratio. In addition, an optically accessible constant volume spray combustion chamber was employed to observe significant differences in the global heat release characteristics and physical/chemical ignition delay times of synthetic alternative jet fuels in comparison to the conventional jet fuels. The results of this study show that synthetic alternative jet fuels with predominantly linear and lightly-branched alkane content (S8, Shell SPK and HRJ8) provide stronger low-temperature ignition characteristics, while other types of synthetic alternative jet fuels with high content of highly-branched alkanes (Sasol IPK and ATJ) exhibit weaker low-temperature ignition characteristics, when compared to conventional jet fuels (JP-8, Jet-A and JP-5). Their unique fundamental ignition behaviors, including the percentage of low-temperature heat release (% LTHR), the critical compression ratio (CCR), and the critical equivalence ratio (phi(cri)(t)), are strongly related to the dominant chemical compositions of the fuel in a motored engine.