This invention refers to a fuel composition identification system and method (2) for an automotive vehicle combustion engine. The system comprises at least a fuel heating resistor (3) arranged in any position in the fuel tank or along the fuel line, in direct contact and exchanging heat with the fuel (2), an electronic control unit (4) connected to the resistor (3), which applies constant power values during time intervals (tn) to the resistor (3), and the metering device (6) for a heating resistor parameter (3) between the current sent to the resistor (3) and the surface temperature of the resistor that sends the measured values to the electronic control unit (4), and monitors the current values measured in the heating resistor (3) during each time interval tn, and when the electronic control unit (4) detects a variation in the value of the heating resistor current in the course of the time interval (tn), it identifies the fuel composition corresponding to the critical heat flow resulting from the power applied to the resistor during that time interval. The method comprises the steps of applying constant power values during time intervals (tn) to the fuel heating resistor (3); heating fuel in contact with the heating resistor (3) through exchanging heat between the heating resistor and the fuel (2); measuring a heating resistor parameter (3) between the current sent to the resistor (3) and the surface temperature of the resistor; monitoring the heating resistor parameter values (3) measured in the course of each time interval (tn), and when a variation in the resistor parameter value is detected in the course of the time interval (tn); identifying the fuel composition corresponding to the critical heat flow resulting from the power applied to the resistor during that time interval.