Acquiring elastic property information from oil shale during pyrolysis remains a difficult endeavor. We subject oil shale samples from the Irati Formation in the Paraná Basin, Brazil, to high-temperature, high-pressure hydrous and anhydrous pyrolysis while simultaneously acquiring P- and S-waveform data. We present elastic properties as a function of temperature at constant differential pressure using samples cored in three different directions relative to the bedding plane. Hydrous pyrolysis decreased total organic carbon (TOC) in samples from 25 wt% to 20 wt%, whereas anhydrous pyrolysis had no significant change in TOC. The cores experienced higher strain hysteresis (up to –0.094 residual axial strain) after hydrous pyrolysis compared to after anhydrous pyrolysis ( residual axial strain). The larger residual strain was accompanied by a larger increase in shear velocity up to 35% after hydrous pyrolysis as compared to the 3% shear velocity increase after anhydrous pyrolysis. The change could be a result of compression and loss of organic content determined at an ambient temperature of 25°C. We quantified sample changes using X-ray computed tomography, scanning electron microscopy, source rock analysis, and X-ray diffraction before and after pyrolysis. Our data on elastic properties and Thomsen’s parameters during pyrolysis can be used to extract material property information useful for improving subsurface well logging and 4D seismic to produce efficiently from oil shale formations.