A new, two-component asphaltene solubility model (ASM) has been developed to predict the phase behavior of asphaltenes in crude oils. Previous asphaltene solubility models have relied for solution upon simplifying assumptions (e.g., nucleation of a pure asphaltene or pure solvent phase) and approximation methods for determining crude oil solubility parameters. No such simplifying assumptions or approximations are needed if a simple correlation between the solubility parameter and the refractive index of nonpolar species is used to characterize the oil. Compositional ranges over which asphaltenes are stable, metastable, or unstable to phase separation can be defined from calculations of changes in Gibbs free energy using the Flory-Huggins theory for mixtures of crude oil and known amounts of hydrocarbon solvents and precipitants. Coupled values of the asphaltene solubility parameter and molar volume represent the only adjustable parameters in the model. Predictions have been tested against an extensive set of onset observations for crude oils and solutions of their asphaltenes. Experimental data consist of microscopic observations of the first appearance of asphaltene aggregates in response to addition of a minimum amount of a hydrocarbon precipitant (with carbon chain lengths varying from n-pentane to n-pentadecane). That first appearance of aggregate corresponds neither to the oil-precipitant mixture in which metastable conditions are first predicted nor to a mixture in which asphaltenes should be completely unstable. A consistent criterion between these two extremes has been defined as the "visible onset." The proposed visible onset criterion has been successfully tested with a wide range of crude oils, precipitants, and solvents.