The complexity of and expense involved in determining the cetane number of diesel fuels using a standard test engine have led to the development and widespread use of the ASTM cetane index. However the cetane index was not intended for use with alternative or unusual diesel fuels; its use with such fuel s can lead to large errors. Using a database of alternative diesel fuel properties, a new cetane number prediction equation has been developed for use with alternative fuels, including alcohols. Compared with the cetane index, the average error in predicted cetane number is reduced by 40% and the number of cases where the error exceeds 5 cetane numbers is reduced by 30%. The significance of the various terms in the equation is discussed.