Acoustic velocities in shales show a relatively strong temperature dependence, which has to be accounted for in the interpretation of time-lapse seismic if temperature changes are anticipated in caprocks or intra-reservoir shales, e.g. in thermal enhanced oil recovery operations or subsurface CO2 sequestration. As shown in the present study, the temperature dependence of ultrasonic velocities in shales can be described by the Gassmann fluid-substitution model by assuming a linear temperature dependence of the rock matrix. The cause for this temperature dependence is not understood yet. SINTEF's shale rock physics model predicts the right trend when assuming a temperature dependence of the bound-water stiffness similar to that of ice. It is also likely that the temperature dependence of velocities is frequency dependent and that temperature changes result in different velocity changes at seismic, sonic, and ultrasonic frequencies.