P wave arrival times have been picked on seismograms recorded during two dual well seismic experiments, separated n time by one year, that were carried out in the Los Alamos Scientific Laboratory Hot Dry Rock Geothermal Energy Reservoir. Average in situ P wave velocity is computed from arrival times measured along segments of the borehole. Velocity decreases as temperature drops owing to heat extraction from the reservoir. The velocity changes linearly with the change in temperature at a rate of 1.07 (km/s)/(100 ¿C). The linear decrease in velocity leads to the important conclusion that P wave velocity can be used as an effective measure of change in in situ temperature during operation of the geothermal system. The change in velocity is attributed to an increase in the microcrack porosity which is a result of the heat extraction from the rock.