Ignition quality of heavy marine diesel fuels is considered to be an important parameter. The standard procedures used to quantify this parameter for distillate fuels are not applicable to residual fuels. Proton NMR Spectroscopy was demonstrated to be an effective tool to characterize the “global” fuel composition of commercially available fuels covering a wide range of ignition quality. Proton NMR data from these fuels were used to determine a cetane number (CNp) for the heavy fuels using procedures previously reported for distillate fuels. The validity of this instrumental technique for determining CNp was corroborated by actual ASTM D 613 engine tests on a number of commercially available fuels, run as blends with secondary reference fuels. Viscosity and density values of the analyzed heavy fuels were regressed against predicted cetane numbers to obtain a correlation expression.