It is defined as the temperature at which equal volumes of aniline and diesel oil are completely miscible. The value gives an indication of the aromatic content of diesel oil, since aniline is an aromatic compound which is dissolved on heating by the aromatics in diesel oil. The greater the aniline point, the lower the aromatics in diesel oil. A higher aniline point also indicates a higher proportion of paraffin.

The diesel index is directly related to aniline point as:
DIESEL INDEX = ((ANILINE POINT(DEG F))(API GRAVITY))/100 A higher aniline point (and therefore a lower aromatic content) in diesel oil is desirable, in order to prevent autoignition in diesel engines