For flight at high Mach numbers, thermal and chemical nonequilibrium may exist in the mean flow and thus affect the stability of the flow. A computational tool was developed to analyze a hypersonic mean flow and its stability including thermochemical nonequilibrium. The mean flow analysis employs the Navier-Stokes equations with a translational/vibrational temperature model for thermal nonequilibrium and a five-species reacting air model for chemical nonequilibrium. The stability analysis employs linear stability theory to describe the spatial amplification of two- and three-dimensional disturbances. The computational tool is used to determine the frequency and spatial amplification of disturbances that may lead to boundary layer transition on cold wall and adiabatic flat plates. The effect of thermal and chemical nonequilibrium on stability is shown to depend on the disturbance mode.