Calorimetric measurements have led to ΔH0values for ionization of nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane in aqueous solution at 298°K. Combinations of these enthalpies with free energies from equilibrium constants for ionization have led to ΔS0values for the ionization reactions. It is noted that the trend toward decreasing pKwith methyl substitution in nitroalkanes is unusual compared to phenols and carboxylic acids. Similarly, correlations of ΔS0with ΔG0and ΔH0are different for nitroalkanes than for other acids.