The speed of ultrasound and density for 3-methylpyridine (1) + heavy water (2) were measured within the whole concentration range and at temperatures from 293 to 313 K. Smoothing polynomials approximating the dependencies of speed and density on temperature were determined. The speeds of sound, densities, and adiabatic compressibilities (calculated using the Laplace formula) were plotted as functions of the mole fraction x(1). The speed of sound and compressibility isotherms show single extrema and nearly common crossing points at low 3-methylpyridine concentrations, while the density decreases monotonically with increasing x(1).