Low temperature isothermal pyrolysis of Thuja (Tetraclinis articulata) wood samples was carried out at atmospheric pressure. Changes in the fixed-carbon yield over time revealed the existence of two distinct phases. The first phase concerned hemicelluloses and cellulose decomposition. During that phase, char formation was promoted by low temperatures. At the end of that phase, the fixed-carbon yield was maximum. The value of the maximum was as great as that obtained under high-pressure pyrolysis and approached the theoretical value. During the second phase of the low temperature isothermal pyrolysis, the fixed-carbon yield decreased, showing a loss of the already existing carbon. But the rise in temperature immediately at the end of the first phase helped to preserve the value of the fixed-carbon yield reached atmaximum. This novel temperature-time profile (low temperature pyrolysis followed by a rise in temperature) constitute a two-step pyrolysis of whole biomass which resolved the dilemma of increasing charcoal quality without excessively decreasing char yield.