The vapor-liquid equilibrium of organic species and water determines the partitioning of volatile organic species between the gas and aerosol phases in the atmosphere. Essential properties that govern this partitioning are the compounds' vapor pressures and the liquid-phase activity coefficients of the organic species. Key issues include (1) the amount of water absorbed by organic particles, both single-component and multicomponent, as a function of relative humidity and (2) how organic and water partitioning is altered as the relative humidity changes, for a fixed total (gas plus particle) quantity of each organic component. We present here calculations relating to each of these issues for organic molecules that are characteristic of those that have been identified in ambient aerosols. Uncertainties exist in knowledge of vapor pressures of atmospheric organics and in activity coefficients, which are calculated by the UNIFAC method. These uncertainties, the level of which is difficult to estimate because of a lack of appropriate thermodynamic data, can lead to significant uncertainty in predicted gas-aerosol partitioning. Current theories for estimating both vapor pressures and activity coefficients are likely to be improved eventually, but the methods discussed here will probably remain the procedures of choice for the immediate future.