In stereochemistry, enantiomeric excess (ee) is a measurement of purity used for chiral substances. It reflects the degree to which a sample contains one enantiomer in greater amounts than the other. A racemic mixture has an ee of 0%, while a single completely pure enantiomer has an ee of 100%. A sample with 70% of one enantiomer and 30% of the other has an ee of 40% (70% − 30%).

Enantiomeric excess is defined as the absolute difference between the mole fraction of each enantiomer:

where

In practice, it is most often expressed as a percentage.

Enantiomeric excess is used as one of the indicators of success of an asymmetric synthesis. For mixtures of diastereomers, there is an analogous definition for diastereomeric excess.

For example, a sample with 70% of R isomer and 30% of S has an enantiomeric excess of 40%. This can also be thought of as a mixture of 40% pure R and 60% of a racemic mixture (which contributes 30% R and 30% S to the overall composition).

If given the enantiomeric excess ${\displaystyle \ ee}$ of a mixture, the fraction of the main isomer, say R, can be determined using ${\displaystyle \ F_{R}=(1+ee)/2}$ and the lesser isomer ${\displaystyle \ F_{S}=(1-ee)/2}$.

A non-racemic mixture of two enantiomers will have a net optical rotation. It is possible to determine the specific rotation of the mixture and, with knowledge of the specific rotation of the pure enantiomer, the optical purity can be determined.