A numerical solution designed to predict the composition of aircraft fuel tank ullage has been investigated. The mathematical model is based on the physical processes governing gas phase diffusion in a stratified ullage. Since an oxygen (O₂) and hydrocarbon (HC) mixture can be combustible, particular attention has been paid to oxygen and hydrocarbon concentration s in the ullage environment. The d iffusion coefficient of the HC mixture was calculated numerically for a range of pressure conditions representing altitudes from sea level to 60,000 ft. Comparisons are given to the calculated mass diffusivity of HC with JP 8, JP 4, and water vapor are described