The invention relates to a device and to a method for determining the proportion of fuel in a combustion engine lubricating oil. The device according to the invention comprises a column (12) with at least 22000 theoretical plates and a steady state phase capable of separating the fuel, the oil, the internal standard contained in the oil and the solvent, in which the oil and the internal standard are diluted, and a heating module (120) capable of causing an increase in the temperature of the column (12) at a rate of at least 350°C/min. In the method according to the invention, the column (12) is subjected to determined cycles during which the pressure of the carrier gas and the temperature are varied.