The radiation chemistry of CO has been studied, using principally low-energy protons from a Van de Graaff accelerator, as a function of dose rate, temperature and small amounts of additives (CO2 and O2). At dose rates higher than those used in previous work a true dose-rate inhibition effect has been found which is interpreted on the basis of competition between second-order and first-order processes. The effect of CO2 on yields is negligible at concentrations up to 0.5% by volume but initial yields are extremely sensitive to traces of O2, particularly at low dose rates. Increased temperature reduces yields of solid and increases yields of CO2, but this effect is controlled mainly by subsequent thermal decomposition of the solid. The empirical composition of the solid is unaffected by changes in dose rate, and varies from (C3O2)n at room temperature to essentially pure carbon at temperatures >450°C. Nucleation of the solid occurs predominantly in the gas phase. The relative importance of ionic and excitation processes is discussed.