Deep eutectic solvents (DESs) are liquid-state mixtures, which mostly consist of two or more solid compounds, at room temperature coming from the strong eutectic effect. After discovering a liquid-state mixture of choline chloride and urea (1:2 in molar ratio) at room temperature as DES in 2003, DESs have gotten more attention recently as a new type of solvent. In this review, we introduce unique DESs consisting of an organic iodide salt, e.g., 1-alkyl-3-methylimidazolium iodides and ammonium iodides, and iodine. The DESs of an organic iodide salt and iodine are easily prepared: just mixing two components. They show a very high electrical conductivity, typically more than 10 mS cm–1. For example, a 1:2.5 mixture of methylammonium iodide and iodine shows approximately 100 mS cm–1 at 298 K. Based on the viscosity and Raman spectrum, the superior electrical conductivity of the DESs is attributed to the Grotthuss mechanism. In this article, we also introduce our attempt for differential calorimetry measurements of the DESs, which possess corrosivity for a wide variety of metals, using a fluororesin-coated aluminum pan.