A method for determining the composition of an alcohol-petrol mixture according to which a light beam is emitted through the mixture and the degree of absorption of this beam by the mixture for at least one wavelength is determined. This wavelength is chosen in the wavelength interval corresponding to the near infrared and the degree of absorption is linked to the alcohol content of the mixture. Such a determination method can be used to automatically adjust operating parameters of a combustion engine supplied with an alcohol-gasoline mixture. To do this, a light beam is emitted through the mixture supplying the motor, the degree of absorption of this beam by the supply mixture in a band of wavelengths chosen in the corresponding wavelength interval near infrared is determined, and the adjustment of the engine operating parameters is controlled as a function of the degree of absorption thus measured. A device for performing such automatic adjustment is also proposed.
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