A general method of thermal analysis is presented, whose aim is to reduce at will pressure and temperature gradients inside the sample submitted to thermolysis. The basic idea is to control the sample temperature so as to keep constant a parameter related to the decomposition rate. Attention is specially called on the case when the controlled parameter is pressure, which allows to monitor at the same time two parameters (pressure and decomposition rate). As an example, one apparatus is described, working in the pressure range between 20 and 10−3 torr. This method of Constant Rate Thermal Analysis (CRTA) appears to be specially suited for thermal analysis under controlled vacuum, for the preparation of well defined porous samples, and for the study of decomposition mechanisms.