The present study is focused on the investigations of the enthalpy of mixing of Sn-rich Fe-Sn liquid alloys by small drops of solid Fe in bulk and nanosized form into the liquid Sn. The measurements were performed at about 1073 K under Ar flow. The enthalpy of mixing has been calculated based on the heat effects recorded after each separate drop and shown positive (endothermic) values indicating that the heat is absorbed during the process. The measured enthalpy consists of the molar enthalpy increment of the added element (heating and melting) and the enthalpy of mixing. Additions of the solid Fe in nanosized form have shown less positive values of the measured enthalpy compared to the additions in bulk form. It relates to the loss of the surface enthalpy of the nanoparticles on dissolution and should be added to the enthalpy increment term. The enthalpy of mixing corresponds to the heat effects by interactions between atoms of the added component and the matrix, both in liquid state. Therefore, this term is independent of the surface enthalpy of the added element. Furthermore, the used Fe nanoparticles were passivated. The thin oxide shell covered the Fe core protecting from the oxidation in the air. Therefore, the enthalpy of the chemical reaction between the oxide shell of the Fe nanoparticles and liquid Sn were considered in the performed calculations as well. The performed microstructure analysis of the samples after calorimetric measurements showed that the intermetallic compounds FeSn and FeSn2 were formed during the solidification of the investigated Fe-Sn samples after the dropping calorimetry experiments. It proves that the dropped Fe pieces where completely dissolved into Sn. The coexistence of these two IMCs in the investigated samples can be explained due to the relative fast cooling of the sample after the calorimetric measurement.