Atmospheric mineral aerosol is a potentially important reactive surface that may provide a heterogeneous sink for gas phase species such as nitric acid (HNO3). We have studied the uptake of HNO3 on Na-montmorillonite, a swelling clay mineral, at low temperatures as a function of relative humidity (RH), HNO3 pressure and clay mass. Condensed phase products were probed with transmission Fourier transform infrared spectroscopy while the loss of HNO3 from the gas phase was simultaneously monitored with mass spectrometry. We find that below 16% RH, uptake of HNO3 on Na-montmorillonite is below our detection limit. However, at 29% RH the lower limit to the uptake coefficient for loss of HNO3 on Na-montmorillonite is γ = 8 ¿ 10-5 as calculated using the BET surface area. This value increases to γ = 4 ¿ 10-4 at 44% RH. The HNO3 content, water content and γ all increase with increasing RH, but are independent of temperature from 210 to 232 K and independent of pressure from 1 ¿ 10-5 to 3 ¿ 10-4 torr HNO3. Surprisingly, at 44% RH, Na-montmorillonite can contain approximately 20% water and 30% HNO3 by weight. The results of this study suggest that swelling clays can impact the partitioning of gas phase HNO3 when sufficient water vapor is available and may serve as a potentially important cloud condensation nucleus in the troposphere.