Abstract The absorption rate and mechanism of NO 2-N 2O 4 gas mixtures diluted with nitrogen into nitric acid solutions were investigated in a wetted wall column at 20 °C and 30 °C. It was found that N 2O 4, which is continuously in equilibrium with NO 2, is preferentially absorbed into diluted nitric acid as well as into concentrated nitric acid. The absorption of N 2O 4 into diluted acid is accompanied by a rapid pseudo-first order reaction with water. Values of H N 2O 4 were measured as a function of the acid strength at 20 °C. Above 63% nitric acid the absorption was found to be purely physical. The solubility of N 2O 4 into concentrated nitric acid (63%) was derived from total vapour pressure data concerning the system NO 2-N 2O 4-H 2O-HNO 3. It was found that Henry's law was valid within the condition considered.