The role of HMPA as a ligand for SmI2 and the stoichiometry and energetics of formation for the SmI2-HMPA complex in THF were investigated employing UV-vis spectroscopy, isothermal titration calorimetry (ITC), and vapor pressure osmometry (VPO). The aggregation number for SmI2 in THF was found to be 0.98 ± 0.09 over the entire concentration range studied (6.71-84.0 mM), indicating that SmI2 is monomeric. The UV-vis data suggest that four HMPA ligands coordinate to SmI2, and this was supported by ITC experiments. The combined results are consistent with [SmI2(HMPA)4] being the reductant responsible for the unique reactivity exhibited by SmI2-HMPA cosolvent combinations.