This report updates and extends previouswork by a statistical analysis of the relationship betweencrude price differentials and three quality differentials,as well as transport costs and seasonal effects. In additionto the API (American Petroleum Institute) gravity number andthe sulfur content of the crudes, which are the qualitiesgenerally included in existing analysis, the report presentsthe impact of acidity (measured by the Total Acid Number -TAN) on the price differential. This is because acidity hasbecome increasingly important as the volume of high acidcrudes, particularly from West Africa, has steadilyincreased in recent years.