Steam distillation is an important mechanism which reduces residual oil saturation during steam injection. It may be the main recovery mechanism in steamflooding of light oil reservoirs. As light components are distilled the residual (initial) oil, the residuum becomes heavier. Mixing the distilled components with the initial oil results in a lighter produced oil. A general method has been developed to compute steam distillation yield and to quantify oil quality changes during steam injection. The quantitative results are specific because the California crude data bank was used. But general principles were followed and calculations were based on information extracted from the DOE crude oil assay data bank. It was found that steam distillation data from the literature can be correlated with the steam distillation yield obtained from the DOE crude oil assays. The common basis for comparison was the equivalent normal boiling point. Blending of distilled components with the initial oil results in API gravity changes similar to those observed in several laboratory and field operations.