Physical factors play a major role in the phenomena involved in the formation and deposition of fuel-insoluble material. Temperature is the most important of these factors and will be dealt with in some depth. Other physical parameters to be discussed include system pressure, flow regime, heat transfer, deposit morphology and physical characteristics, and surface roughness of the test section. In addition, some recent efforts in modeling the overall process of thermal oxidation stability will be addressed in this chapter although the models include chemical as well as physical effects. The variety of equipment used in thermal stability work and the multiplicity of goals behind this type of research may explain the different results and conclusions reached by different investigators. If a reasonable explanation can resolve a difference in results, this will be presented.