Experiments were performed to obtain histories of surface temperatures and rates of upward flame spread for vertically oriented, thermally thick wood slabs exposed to surface fluxes of thermal radiation up to 2.6 W/square centimeters. Above a critical irradiance sustained upward flame spread occurred for Douglas-fir particle board with pilot initiation at the base of the fuel face. Data obtained included temperatures, flame heights, pyrolis-front heights, combustion duration, and char-layer thickness for various irradiances and preheat times. The measurements were compared with theory.