Launching the next generation of satellites, producing high-fidelity nano-materials, and driving blast waves all rely on complex phase change and rapid chemical reactions. However, the coupling of these processes as well as the fundamental reaction kinetics is poorly understood owing in large part to the difficulty in observing the highly reactive phenomena on relevant time and length scales. In this talk, I outline a multi-method approach for applying optical diagnostics and novel modelling techniques in complex operating environments to identify key drivers of reactivity, including multi-phase interactions and key gas-phase reaction mechanisms. Key to many of these processes is the high-temperature gas-phase reactivity of base fuels or stable intermediates, with the potential for reaction mechanisms defining millisecond scale phenomena to encompass hundreds of possible reactions. I will highlight the role of highly repeatable shock tube experiments with high-fidelity diagnostics in unlocking the fast kinetics (<10 µs) behind these complex processes. In addition to highlighting the role of temperature on the gas-phase chemistry, I will also focus on the role of pressure in driving the multi-phase reactivity and phase change in propellant and energetic material applications.