The present day measure of the ignition quality of a diesel fuel is the cetane number. Cetane number determination is carried out using a special single cylinder engine with reproducible operating conditions and variable compression ratio. The importance of the carbon skeletal structure of the fuel on the ignition quality is qualitatively well known, but the practice of defining the ignition quality of diesel fuels by a term, whose physical and/or chemical meaning is not well understood, has not been abandoned yet. The correlations that have been proposed recently, which relate the total fuel aromaticity or mid-boiling point, hydrogen content and density to the cetane number, suffer from the lack of representation of the fuel's compositional structure, and of well defined relationship, if any, between boiling point, hydrogen content, density and ignition quality. Recent work in our Laboratory has shown that 1H NMR (proton Nuclear Magnetic Resonance) spectroscopy is best suited to obtain the average carbon group type structural composition of the engine fuels. Although, 1H NMR has been widely used to characterize various petroleum and other liquid fuels, there has been no attempt to correlate the complete carbon group type structure to the ignition quality. In the present work, a large number of diesel fuels with wide ranges of cetane numbers and hydrocarbon type composition have been analyzed, and correlations have been derived which yield expressions for cetane number as a function of carbon type structure of the fuel.