The paper presents a computational and an experimental investigation of base flow of a circular cylinder at hypersonic speeds. Effects of chemistry and wall temperature on the flow in the base region, at low to high enthalpies, are discussed. The experiments were conducted in a shock tunnel at a nominal Mach number of 10. Freestream Reynolds numbers based on cylinder diameter were 0.97 x 10(4) and 3.74 x 10(4), respectively, and the total specific enthalpies were 13.35 and 3.94 MJ/kg, respectively. The test gas was air. The surface pressure and heat flux were measured using a cold wall model. Equilibrium and thermal as well as chemical nonequilibrium numerical simulations were performed using a Navier-Stokes equations-based computational fluid dynamics code. Both a cold wall and adiabatic wall were considered. Particular emphasis was placed on the wake structure, vorticity distribution, wake centerline aerothermodynamic properties, and surface data. The existing low-enthalpy cold hypersonic windtunnel experimental data are included for comparison. The simulations predicted the effect of chemistry on the near wake to be negligible for the low-enthalpy, high Reynolds number flow but more significant for the high-enthalpy, low Reynolds number flow.