The thermal decomposition of solid sodium bicarbonate has been studied in the temperature range 360–500 K over a range of partial pressures of carbon dioxide. The effect of water vapour has also been studied. Above 440 K the reaction follows contracting-cube kinetics with an activation energy of 32 kJ mol^–1 and a frequency factor of 10¹ s^–1. In this temperature range the presence of water or carbon dioxide has little effect on the kinetics. Below 390 K the reaction follows first-order kinetics. In nitrogen, the activation energy is ca. 64 kJ mol^–1, the frequency factor is 10⁵ s^–1 and water vapour has little effect. High partial pressures of carbon dioxide increase the activation energy to ca. 130 kJ mol^–1 and the frequency factor to 10^13.5 s^–1. The results of microscopic examination generally confirm the kinetics but show that at low temperatures in nitrogen and carbon dioxide the processes are different in detail.