Ultrasonic energy has been widely used to disrupt soil aggregates before fractionating soil physically when studying soil organic matter (SOM). Nevertheless, there is no consensus about the optimum energy desirable to disrupt the soil. We therefore aimed (i) to quantify the effect of varied ultrasonic energies on the recovery of each particle-size fraction and their C, N and δ13C distribution, and (ii) to determine an ideal energy to fractionate SOM of a specific soil. Our results show that the 2000-100 ?m particle-size fraction was composed mainly of unstable aggregates and the 100-2 ?m fraction of stable aggregates. Energies of 260-275 J ml-1 were sufficient to disrupt most of the unstable aggregates and leave stable aggregates. The use of this threshold energy combined with particle-size fractionation was not satisfactory for all purposes, since litter-like material and relatively recalcitrant organic carbon present in stable aggregates > 100 ?m were recovered in the same pool. An ultrasonic energy of 825 J ml-1 was not sufficient to stabilize the redistribution of soil mass and organic matter among particle-size fractions, but at energies exceeding 260-275 J ml-1 relatively stable aggregates would fall apart and cause a mixture of carbon with varied nature in the clay fraction.