The Performance of ZrB2-SiC ultrahigh-temperature ceramics in subsonic high-enthalpy dissociated airflows was investigated in the 1.2 MW plasmatron facility at the Von Karman Institute for Fluid Dynamics. Samples were sequentially exposed to low and high heat flux conditions during a single test run. Under certain high heat flux conditions, a rapid and spontaneous rise in sample surface temperature was observed under constant freestream conditions. The temperature jump seems associated with a transition in surface chemistry that leads to the loss of protective silica glass and substantially increases the chemical component of heat flux delivered to the surface.