The formation of secondary organic aerosol and gas/particle partitioning of carbonyl products from the photooxidation of p-xylene has been investigated as a function of relative humidity. Experiments were performed in an atmospheric simulation chamber at atmospheric pressure and ambient temperature in the presence of NOx. Aerosol yields increased by a factor of approximately two over the relative humidity range 5-75% and were found to correlate with initial water vapor concentration and hydroxyl radical (OH) concentration. The results indicate that an increase in relative humidity results in higher levels of HONO formation in the chamber which leads to increased OH concentration, a faster p-xylene decay rate, and higher aerosol mass yields. A recently developed denuder-filter sampling technique was used to investigate the gas/ particle partitioning behavior of the carbonyl photooxidation products. The identified products accounted for up to 18% of the aerosol mass formed. Dicarbonyls with at least one aldehyde functionality (e.g., glyoxal and methylglyoxal) exhibited gas/ particle partitioning coefficients several orders of magnitude higher than expected from absorptive partitioning theory, suggesting that reactive uptake and particle phase reactions are important processes for aerosol formation from these species. Experimental gas/particle partitioning coefficients were also found to be dependent on relative humidity, with every dicarbonyl exhibiting much lower values when the relative humidity was increased from 50% to 75%.