A computational fluid dynamics technique is developed for the study of high-pressure, axisymmetric hypersonic nozzle flows. The effects of intermolecular forces and vibrational nonequilibrium are included in the analysis. The numerical simulation of gases with an arbitrary equation of state is discussed. Simulations for high-pressure nitrogen nozzles (P0 = 138 MPa) demonstrate that both intermolecular forces and vibrational nonequilibrium affect the flow significantly. These nonideal effects tend to increase the Mach number at the nozzle exit plane. In addition, these effects may introduce weak expansion and compression waves in the nozzle that degrade test section flow quality. Thus, they must be included in the design and analysis of high-pressure hypersonic nozzles.