Abstract—At 298.15 K, ultrasonic speeds and the isentropic compressibilities of aqueous solutions containing formamide (F) + t-butanol have been measured. As the concentration of formamide in the cosolvent (aqueous formamide) increases, it is observed that the concentrations of t-butanol at which isentropic compressibility becomes minimum and ultrasonic speed becomes maximum decrease. This behavior suggests that compared to aqueous t-butanol, aqueous ternary solutions are less structured. The concentration dependence of excess isentropic compressibility and excess ultrasonic speed very nicely reflects this behavior. This behavior is explained by the fact that the presence of formamide reduces
t-butanol's capacity to form clathrate hydrates. Isentropic compressibility rises with concentration and ultrasonic speed falls when formamide concentration in the cosolvent (xF) > 0.2.