A CFR engine has been used to 1. Develop a test procedure to determine the influence of the fuel composition on Octane Requirement Increase (O.R.I.), 2. Examine the mechanism of this phenomenon. A short duration test procedure of 35 hours using a small quantity of fuel is proposed. Testing of several gasoline compositions has indicated reasonable repeatability and discriminating power between these gasolines.The mechanism proposed for O.R.I. combines two factors: First, thermal measurements have shown the insulating properties of the deposit layer, formed in the combustion chamber, which causes an increase in the temperature of the end-gas thus resulting in earlier self-ignition. Secondly, the occurrence of deposits on the intake valve produces an enhanced swirl. This factor and the increase of the temperature of the charge cause an acceleration of the flame speed. Computations show that the earlier combustion due to this acceleration contributes to the O.R.I.