Research and Motor Octane Number (RON and MON, respectively) measurements were performed to assess a linear-by-mole computation method for octane number prediction based on pure compound values. Measurements demonstrated that a linear computation method is accurate for ternary mixtures of n-heptane, iso-octane and toluene, with a very high level of accuracy for RON, MON and sensitivity in the range of values of actual gasolines. The relation was further compared to alternative formulations from the literature to demonstrate its relevance and better accuracy despite its simple form. Such a relation is a key to define surrogates for gasoline when representativity for auto-ignition is looked after. (C) 2013 Elsevier Ltd. All rights reserved.