This work analyzes the thermodynamic behavior of two candidate aluminum alloys (A355 and B319) for SSM processing. Both alloys undergo a semisolid tertiary phase transformation below the main eutectic solidification temperature. The use of the differentiation method (DM) to identify critical points associated with the thixoformability of these alloys is evaluated. The method is compared with the conventional tangent method (TM) for determining the solidus and liquidus temperatures and with the visual recognition method for identifying the unstable "knee" of the liquid fraction-versus-temperature curve. For both alloys, numerical simulation is performed with Thermo-Calc (R) (Scheil condition), and experimental DSC data is generated for cooling (rheocasting) and heating (thixoforming) cycles under different kinetic conditions (5, 10, 15, 20 and 25 degrees C/min) to compare the DM and the conventional approaches. The findings indicate that the DM is an efficient tool for identifying critical points such as the solidus, liquidus, knee and beginning and end of the SSM-processing working window for A355 and B319 alloys and that it not only yields results that agree well with those obtained using traditional techniques, but also, unlike the latter, provides accurate results even in the presence of tertiary phase transformations when the kinetic effects are strong. A correction to the Flynn method of integration of partial areas using two complementary baselines is introduced for rheocasting and thixoforming operations.