Ultrasonic speeds have been measured for binary mixtures of N-methyl-2-pyrrolidinone with 2-propanol, 2-methyl-1-propanol, 2-methyl-2-propanol, and 3-methyl-1-butanol, as a function of composition at 298.15 K. The values have been combined with those of densities derived from excess molar volumes to give estimates of the product K S , m , of the molar volumes and the isentropic compressibility K S and the excess quantity K E S , m . The K E S , m values are negative over the entire range of composition for all mixtures. The deviations u D of the speeds of sound from their values in an ideal mixture were also evaluated for all measured mole fractions. Graphs of u D , and K E S , m against composition are presented as a basis for a quantitative discussion of the results. Various thermodynamic parameters have been derived from the speed of sound data. Theoretical values of speeds of sound u have also been determined using empirical relations.