The semiconductor properties of cadmium sulphide are known for a long time, but at present its photocatalytic and photoemission of electrons properties are studied intensively. On the other hand the cadmium sulphide is the most important additionally sulphide in zinc blende and from this position is an important source for obtaining cadmium. Thus, the study of CdS oxidation is a very important problem for the optimization of its roasting process and also for to know its resistance to oxidation at use in photovoltaic cells, photocatalysts or light sensors. The oxidation of CdS to CdO is a complicate process that has generated controversial opinions on mechanism. The main theories, oxide formation or sulphate formation in the first stage, agree both that the final stage of process is the CdSO4 decomposition from a CdO:CdSO4 = 2:1 mixture. Our study on CdS, CdO:CdSO4 = 2:1 mixture (obtained from CdO p.a. and CdSO4 p.a.) and CdSO4 p.a. using non-isothermal TG/DTG-DTA method coupled with mass spectrometry on CdS has confirmed the existence of CdO:CdSO4 = 2:1 in the final stage of CdS oxidation and the CdSO4 decomposition (see Fig.1). The apparent activation energy-Ea and preexponential factor-A were determined using the ASTM E 1641-04, “Standard Test Method for Decomposition Kinetics by Thermogravimetry”, based on Flyn-Wall isoconversional method. Also, Ea and A were determined for the mixture of CdO:CdSO4 = 2:1.and for CdSO4 separately. The compensation effect was observed for Ea and A (see Fig.2). Fig.1. TG curves vs. time for CdSO4, CdO:CdSO4 Fig. 2. LnA vs. Ea for α=5, 10, 15 and and CdS for the heating rate of 5 K/min. 20 % Thus, although the Ea has the highest value (355±6 kJ/mol) for CdS and the lowest (291±4 kJ/mol) for CdSO4, the rate constant-k increases in order: CdSO4< CdO:CdSO4 = 2:1< CdS(CdO:CdSO4) and as result the conversion to CdO is completely in a shorter time for CdS.