The acoustic emission of Westerly granite sujected to temperature changes up to 120 ¿ at various heating rates from 0.4 to 12.5 ¿C/min was studied. A threshold temperature of 60 to 70¿ c appeared to exist for this range of heating rate, above which A.E. began to occur profusely with increasing temperature. Above the threshold temperature, the rate of A.E. depended strongly on the rate of heating. However, a thermal' Kaiser' effect appeared to exist such that in cyclic heating and at temperatures below the maximum temperature reached in the previous cycle, very few A.E. occurred irrespect of the heating rate in the subsequent cycles. We suggest that there is an equilibrium state of the thermally induced cracks, which is a function of temperature but is independent of the heating rate; in the slower experiment, the equilbirium state of crack extension is achieved by slower growth, which is associated with smaller amounts of acoustic emission.