Petrophysical experiments on two Icelandic geothermal rock samples at simulated in situ reservoir conditions are analysed to delineate the effect of temperature on seismic velocity and attenuation. A goal of the present work is to predict the effect of the saturating pore fluid on seismic velocity using the Gassman equation, which has been modified for this purpose. To include the temperature effect in the equation, two assumptions are made: (1) the grain/mineral and dry bulk moduli are independent of temperature; and (2) the temperature dependence follows solely from the thermophysical characteristics of the saturating fluid through the fluid bulk modulus and fluid density. Laboratory measurements show that P-wave velocities decrease with increasing temperature. This change is related to the thermophysical characteristics of the saturating fluid; at higher temperatures bubbles and thermal microfractures are formed affecting seismic velocities. The measurements also show that at low temperatures seismic attenuation decreases with temperature due to the rapid decrease in the fluid viscosity. On the other hand, at higher temperatures the attenuation increases because of the generation of bubbles and thermal microfractures. Although having data from only two samples and that no measurements on dry samples were done, thus limiting the generality of the claims that can be made, the study presents a plausible approach to relate changes in seismic properties to geothermal system temperatures. (c) 2009 Elsevier Ltd. All rights reserved.