1. The shift in the ultraviolet absorption maximum of 2-nitropyrrole resulting from change in solvent from glacial acetic acid to dilute acetic acid is associated with hydration of the solute molecules; the heat of hydration has a value of from −1.5 to − 1.7 kcal/mole. 2. In the course of the investigation measurements were made on the heat of mixing of acetic acid and water at 21–22‡ for final concentrations of 25–35%. The heat of mixing is zero when the acetic acid content of the final mixture is 28%.