We report on the occurrence of exothermic phenomena during the thermal ramping of mesoporous silicon powders in ambient air. For furnace set temperatures of up to 800 °C, discrete exotherms occur during the initial ramp-up stage from room temperature. With an onset around 200 °C, the powder temperature rapidly self-elevates to significantly beyond the concurrent furnace baseline value and cools thereafter, in most cases over a period of a few minutes. A number of periodically spaced exotherms can occur, depending on both the weight and layout of the powder batch. A broadening and amalgamation of exotherms are observed for large batch sizes, indicating a longer-term retention of induced thermal energy, in one case with the powder temperature rising well beyond 1000 °C and being maintained for up to 80 min. We associate these exotherms with dehydrogenation processes, some of which may involve N–H as well as Si–H bonds. Oxidation is accompanied by the release of silanes and hydrogen, care therefore being required to avoid combustion of these pyrophoric gases.