The photolysis of nitric acid has been studied, at 2650 Å at room temperature, in the presence of H₂ and CO, respectively. It was concluded that HNO₃ decomposes according to a chain mechanism in the presence of these gases. The mechanism has been discussed in detail. The results made it possible to determine rate constants for the reactions of nitric acid with hydroxyl radicals and hydrogen atoms, respectively. The rate constants obtained from different experiments were compared with one another and with the data available in the literature.