The velocity of ultrasonics in compressed gases was measured by the optical method using the diffraction of monochromatic light. Our investigations may be summarized as follows: (1) Measurements on various compressed gases (A,N2, CH4) at pressures up to 1200 atmos, temperatures from 20°C to 200°C, and at frequencies ranging from 0.5 to 3.5 Mc. No definite dispersion was found in that range. The velocity increases as a function of pressure; at high densities its temperature coefficient is negative. The velocities calculated from the experimental PVT data of other authors are in good agreement with our measured velocities. The ratio of specific heats and the adiabatic compressibilities have been calculated. Calculations have been made for A,N2, andCH4using a series expansion of the velocity of sound with respect to densities; the coefficients of this expansion are related to the first four virial coefficients and their first two derivatives. The usual (6–12) spherically symmetric intermolecular potential has been assumed. It is found that a satisfactory agreement between calculated and observed velocities can be achieved at densities up to 300 amagat units; this agreement is obtained for both PVT and sound velocity data, by taking into account only a constant value of the fourth virial coefficient in our temperature range. It is concluded that a purely thermodynamical interpretation is sufficient to explain the observed velocities in our range and that sound velocity data seem a convenient means to test, jointly with the equation of state, the accuracy of a given intermolecular potential law. (2) Measurements of ultrasonic velocity near the critical point: The decrease of the velocity, previously observed forCO2andN2O, has been studied inC2H6andC3H8. Systematic measurements onC3H8show that no dispersion is present in our frequency range (0.585 to 3.2 Mc). Values of the velocity have been calculated on a purely thermodynamical basis from some semiempirical equations of state. The measured values are a little lower than the calculated values in the immediate vicinity of the critical point. Such a possible disagreement has been tentatively ascribed to the effect on the bulk viscosity of a structural relaxation process. (3) Measurements by a pulse method of the ultrasonic attenuation in nitrogen at pressures from 80 to 1400 atmos are presently in progress. Preliminary results indicate that the attenuation is strongly decreased under pressure.