We have developed a comprehensive experimental capability to study thermally induced microfractures in rock. We count acoustic emissions and measure compressional wave velocities while a sample is heated to a maximum temperature of 300 ¿C under confining pressures up to 55 MPa. After a heating cycle we measure crack porosity and crack compressibility. We count the cracks under an SEM, sorting them by grain boundary and intragranular types and by mineral pair occurrence. Our observations reveal that (1) significant thermal cracking occurs in granites heated above 200¿ or 250 ¿C under 28 and 55 MPa and above 100 ¿C in granites heated under 7 MPa, (2) most newly created thermal cracks close at pressures below 40 MPa (1.5 km depth), (3) grain boundaries between quartz grains or between quartz and another mineral are preferentially cracked, (4) the threshold temperature for acoustic emissions is a positive, linear function of pressure, (5) total acoustic emission counts in Westerly granite heated to 300 ¿C are slightly lower under 55 than 28 MPa confining pressure and are substantially lower under 28 than 7 MPa, (6) the compressional velocity decreases 22% as Westerly granite is heated to 300 ¿C at 7 MPa, 15% at 28 MPa, and 10% at 55 MPa, and (7) greater hysteresis in compressional velocities occurs in samples heated at lower pressures. The acoustic emission counts in Westerly granite during heating show one or more peaks of activity as a function of temperature, with more peaks at lower pressures. We interpret this result to mean that additional crack populations are able to open at lower confining pressures. The curves of acoustic emission count rates can be fit by summing one to three Gaussian density functions. ¿ American Geophysical Union 1989