The autoignition process of different propane mixtures was studied to determine how the diluent choice (helium and argon/nitrogen) affects the ignition delay, the production of excited radicals, and the overall combustion process. An optically accessible rapid compression machine, high-speed camera, flame spectrometer, and numerical model were used to investigate the combustion process. The ignition delay times were measured at a wide range of compressed gas temperatures from 860 to 950 K, a compressed gas pressure of approximately 34 bar, and two equivalence ratios of 1.0 and 1.5. Chemiluminescence at wavelengths of 309, 555, 590, and 623 nm from the decay of excited radicals was detected during the autoignition process, and their respective intensities were measured and compared at various test conditions. A new kinetic model was built by combining reaction rates of excited radicals and species from the literature. The new model was validated against the measured ignition delay data of this work. The ignition delay and some of the excited radicals show a strong dependency on the diluent choice. A new numerical model was used to include the effect of heat transfer and the diluent choice in the temperature calculation and ignition delay modeling and measurements. Using the new numerical model, we found that the measured ignition delays are not dependent on the diluent choice.