Abstract: An air/fuel ratio control system for internal combustion engines performs .lambda. control by using .alpha. correction coefficients variable depending upon oxygen concentration in the exhaust gas. The system derives an average value of the .alpha. correction coefficients for a predetermined time after the engine enters an idling state while the engine driving condition satisfies a predetermined feedback control for performing .lambda. control, and modifies the average value by adding a predetermined value determined on the basis of an engine coolant temperature. The modified average value is set as a fixed value and is substituted for the .alpha. correction coefficient after the predetermined time elapses.