This work was intended to explore the reliability of the Newtonian thermal analysis method as a tool for the quantification of the amount of alloy microconstituents formed during solidification as observed at room temperature in alloy systems without phase transformations in the solid state. This technique was applied to three hyper eutectic Pb–Sn alloys with different primary phase content using different approaches to determine the time of start of solidification and for the zero curve calculation. Quantitative metallography was used to determine the primary phase content in the experimental probes. The outcome of this work shows that the use of Newton method for prediction of the amount of microconstituents formed during solidification obtain good results in two of the three cases under study suggesting that this method is not reliable.