The ignition of a combustible liquid near a heated vertical surface was investigated theoretically and experimentally for laminar natural convection conditions. Only pressures greater than the critical pressure were considered, where the combustible is a single-phase fluid. Analysis of ignition involved solution of the nonsimilar boundary layer equations for flow along the surface and in the plume, allowing for the nonideal properties of supereritical fluids. Ignition was prescribed as the surface conditions which separated regimes where decomposition waves either quenched or developed in the plume. Experiments were conducted with ethylene oxide at pressures of 5.50-19.30 MPa. Predictions could be matched to the measurements for supereritical conditions, using global first-order kinetics. The results indicate a slight increase in ignition resistance as pressure is decreased, for first-order kinetics, due to property variations near the critical point. Limited measurements at subcritical conditions indicated that ignition occurs when the heat flux exceeds the critical heat flux for nucleate boiling, at pressures somewhat below the critical pressure, since the minimum surface temperature for ignition is not accessible for conventionally heated surfaces in this region. This change in ignition mechanism is probably responsible for the unusual variation of ignition properties frequently observed near the critical pressure of combustibles.