Depending on the source, bitumen can be characterised by being either paraffinic or naphthenic. Naphthenic bitumen tends to be more acidic as it contains naphthenic acid and has acid values of around 1 - 2 mgKOH/g. The acidic components provide a range of benefits, such as improved adhesion, emulsion formation and emulsion breaking properties.

Paraffinic bitumen contains relatively few acidic components and therefore has acid values of around 0 - 1 mgKOH/g. The lower acidity can result in reduced adhesion of the bitumen to the aggregate and poor emulsion properties. To obtain the beneficial properties naphthenic bitumen provides, the acidity needs to be improved.