The size distributions of organonitrate functional groups in ambient Houston, TX aerosol were determined at 3 sites during an air quality field study conducted during August and September 2000. Samples were collected using a Hering low-pressure impactor and were analyzed, in transmission mode, using Fourier transform infrared spectroscopy. The size distributions of organonitrate groups generally fell into 4 categories. In approximately 25% of the samples, the majority of the organonitrate group mass was found in aerosol of approximately 0.1 m aerodynamic diameter. In approximately 26% and 12% of the samples, the majority of the mass was found in aerosol of approximately 0.25 and 1 m diameter, respectively; 21% of the samples displayed both 0.1 and 1 m size modes. The remainder of the samples had relatively low organonitrate mass or other size distributions. Total organonitrate group absorbances, per m3 of air sampled, were generally similar to measurements from samples collected in Los Angeles in previous studies, however,some samples had organonitrate absorbances that were an order of magnitude more intense than observed at urban sites in Los Angeles. The events with high organonitrate concentrations were observed at source-dominated (industrial) sites and were not correlated with either high NOx or high ozone concentrations, suggesting that the high concentrations are due to primary sources. The high organonitrate concentrations were generally accompanied by evidence of high acidity and high organic concentrations.