Ultrasonic speeds have been measured at 298.15 K and 308.15 K for mixtures of formamide + 1-propanol or 2-propanol. For an equimolar mixture, excess molar compressibility follows the sequence of 1-propanol > 2-propanol. The ultrasonic speed data are correlated by various correlations such as Nomoto's relation, van Dael's mixing relation and impedance dependence relation, and analyzed in terms of Jacobson's free length theory and Schaaff's collision factor theory. Excess isentropic compressibility is calculated from experimental ultrasonic speed data and previously reported excess volume data. The excess molar ultrasonic speed and isentropic compressibility values are fitted to Redlich-Kister polynomial equation. Other properties such as molecular association, available volume, free volume, and intermolecular free length are also calculated. The excess isentropic compressibility data are also interpreted in terms of graph theoretical approach. The calculated isentropic compressibility values are well consistent with the experimental data. It is found that the interaction between formamide and propanol increases when hydroxyl group attached to a carbon atom has more -CH3 groups. (C) 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.