A new spectroscopic predictor for estimating ignition characteristics including derived cetane number is proposed for hydrocarbon fuels. This spectroscopic predictor is the ratio of room temperature absorbance of unreacted fuel vapor at 3.41 and 3.39 mu m, termed here as the "3.41/3.39 absorption ratio." Its wide availability and applicability are demonstrated for a range of pure hydrocarbons, mixtures of pure hydrocarbons, and distillate and synthetic jet fuels. Quasi-linear calculation methods are provided for practical use. Spectroscopic and kinetic interpretations are provided based on the fraction of -CH2- hydrogen relative to all carbon-bonded hydrogen. In addition, the correlations between the proposed predictor and ignition delay time and C2H4 yield are presented and discussed to exhibit the predictor's potential as a fuel screening tool.