When air is passed through a hot graphite channel, the heat produced by the chemical reactions is due to both the C-O2 and CO-O2 reactions. The data show that the largest and most rapid temperature rises are due to the CO-O2 gas phase reaction. Serious instability (where the heat generated by the reactions is greater than the heat removed by the air stream) does not occur below 650°C and is confined to flow rates where the Reynolds numbers lie between 2000 and 8000. Although the experiments were designed to provide information for operation of the BNL Reactor, the results were found to be general in nature.