The usefulness of differential thermal analysis can be extended considerably by controlling the composition of the furnace atmosphere. The method described for securing atmosphere control is applicable to most existing differential thermal analysis furnaces. Examples are given which show the effect of a furnace atmosphere of nitrogen on differential thermal curves of clays containing organic matter and pyrite. Other curves show the effect of CO2 on differential thermal curves of siderite, magnesite, dolomite and calcite. One curve follows the alternate dissociation-reconstitution of the CaCO3 part of dolomite in an atmosphere of CO2. The effect, on the differential thermal analysis curve, of filling the furnace with a gas which is a participant in the reaction is explained by the relation of the partial pressure to the equilibrium constant and the heat of reaction.