In the present work the effect of acetic and tartaric acid upon the thermal decomposition of CaCO, has been studied. Mixtures of CaCO, and 5, 10 and 20% acetic and tartaric acid have been prepared. These mixtures were heated at various temperatures in order to study the progress of calcium carbonate decomposition by loss on ignition measurements. Differential Scanning Calorimetric as well as XRD studies have demonstrated that the addition of acetic or tartaric acid to CaCO, promotes its themal decomposition. V. Kas~Aouri et al. included three mixtures 20% warm water. Vol. 2.5, No. 3 from CaCO, with 50, 10 and 20% w/w acetic acid in the presence of The mixtures of the second series were prepared from CaCO, with 5, 10 and 20% w/w tartaric acid in the presence of the same amount of warm water. The above mentioned amount of the water added was similar to that existing in the semi-dry process for the cement production. The following studies have been made on the prepared mixtures. a. Loss on ignition measurements. b. X -Ray Diffraction (XRD) Study of the samples heated at various temperatures. c. Differential Scanning Calorimetry (DSC) measurements of absorbed heat during the thermal treatment of the mixtures. The XRD patterns concerning the mixtures have been prepared and thermaly treated, are presented in the Figures 1, 2, 3 and 4. The various crystalline compounds identified through XRD analysis at different thermal levels, are given in Table I. TABLE I. The formation of crystalline compounds during the burning of CaCO, -acic mixture Sample Crystalline compound Thermal area "C (CH,COO), Ca.0.5 H,O 20 -250 3(CH,COO), Ca2CH, COOH 20 -250 CaCO, + CH,COOH f3 -(CH,COO),