A method is provided for biodiesel blending detection in an internal combustion engine that includes, but is not limited to a first evaluation of the relative air-to-fuel ratio (RAFR) by means of a first sensor whose output whose output is representative of the actual RAFR value, in order to use such first evaluation as a reference value, a second evaluation of the relative air-to-fuel ratio (RAFR) performed measuring mass air flow (MAF), injected fuel quantity (Qfuel) and stoichiometric air-to-fuel (A/F)ST ratio of petrodiesel and carrying out said second evaluation by means of the Electronic Control Unit (ECU) of the engine, and determining discrepancies of values obtained from the second evaluation compared with values obtained from the first evaluation.