Abstract The velocity and absorption of second sound in dilute 3 He- 4 He mixtures under pressure have been measured by a heat-pulse, time-of-flight technique. In order to eliminate path-length effects, the velocity of second sound was determined by subtracting the simultaneously measured times of flight of single second-sound pulses over two different path lengths. At overall pressures between p = 0.0 MPa and p = 2.0 MPa, tabulated values of the second-sound velocity are presented, in pure 4 He in the temperature range between T = 0.7 K and T = 1.1 K, and in dilute 3 He- 4 He mixtures in the temperature range between T = 0.2 K and T = 1.1 K at molar 3 He concentrations between X = 5.5 × 10 -4 and X = 1.73 × 10 -3 . Values of the absorption coefficient, deduced from the broadening of the transmitted second-sound pulses, are given, in pure 4 He in the temperature range between T = 0.7 K and T = 1.1 K, and in dilute 3 He- 4 He mixtures in the temperature range between T = 0.35 K and T = 1.1 K at a zero-pressure molar 3 He concentration of X = 9.2 × 10 -4 . From the data in pure 4 He, values of the phonon and roton parameters were calculated. From the data in dilute 3 He- 4 He mixtures, values of the 3 He contribution to the normal-fluid density were calculated. The values of the effective mass of the 3 He quasiparticles, determined from these values, are in excellent agreement with results of other experiments.