Oil composition changes during production, transportation, and processing of crude oils. The effects of compositional changes on asphaltene stability have been investigated for different crude oils, fractionated asphaltenes, and resins with hydrocarbons that act either as asphaltene solvents or flocculating agents. The refractive index of onset mixtures is used to quantify solution properties with respect to asphaltene stability, and the results are compared to predictions of an asphaltene solubility model. Asphaltene aggregates in depressurized crude oil samples can be explained in terms of natural flocculants in the oil. The effects on asphaltene stability of adding aromatic solvents to an oil have been investigated. Mixtures of asphaltenes and resins show predictable changes in asphaltene stability. The results of this study have important implications for asphaltene stability testing using dilute oil solutions and can be interpreted without any assumptions about special interactions that are often assumed to exist between asphaltenes and resins.