Modern petroleum processing operations require improved methods for the characterization of the heavy ends with regard to the types of hydrocarbons and nonhydrocarbons. The development of investigational tools for this characterization is one of the objectives of API Research Project 60. The project has developed a scheme for separating the highest boiling cuts of crude oil into acids, bases, neutral nitrogen compounds, saturates, and aromatic fractions. This scheme has been successfully tested on high boiling cuts of five widely varying crude oils, and the results of these tests are presented. Results of additional characterization of the acid, saturate, and aromatic fractions are reported. Tests are underway of a modified separation procedure that will subdivide the aromatic fraction and allow more discrete characterization. Progress has also been made in the separation, classification, and understanding of the metallo-organic compounds.