The heterogeneous reaction of N<sub>2</sub>O<sub>5</sub> with airborne illite and Arizona test dust (ATD) particles was investigated at room temperature and at different relative humidities using an atmospheric pressure aerosol flow tube. N<sub>2</sub>O<sub>5</sub> at concentrations in the range 8 to 24 × 10<sup>12</sup> molecule cm<sup>−3</sup> was monitored using thermal-dissociation cavity ring-down spectroscopy at 662 nm. At zero relative humidity a large uptake coefficient of N<sub>2</sub>O<sub>5</sub> to illite was obtained, γ(N<sub>2</sub>O<sub>5</sub>) = 0.09, which decreased to 0.04 as relative humidity was increased to 67%. In contrast, the uptake coefficient derived for ATD is much lower (~0.006) and displays a weaker (if any) dependence on relative humidity (0–67%). Potential explanations are given for the significant differences between the uptake behaviour for ATD and illite and the results are compared with uptake coefficients for N<sub>2</sub>O<sub>5</sub> on other mineral surfaces.