The infrared radiation of a pulsed Nd:YAG laser is employed to generate, either by electrostriction or by thermalization of absorbed laser energy, a spatially periodic density grating that oscillates in time. The second-harmonic output of the same laser is injected into a high-reflectance optical cavity, and the pulse trapped in the cavity is used to monitor the temporal evolution of the grating diffraction efficiency. The oscillation period of the diffraction efficiency depends on the sound velocity in the medium. If the gas composition is known, measurement of the sound velocity allows the temperature to be deduced. On the other hand, if the temperature is known, concentrations in isothermal binary mixtures can be determined. We demonstrate the applicability of this novel one-laser grating arrangement by concentration measurements in a cell containing methane-nitrogen mixtures and by preliminary temperature measurements in a premixed methane-air flame.