The thermal elimination reaction that appears in some polycarbonates has been studied by density functional theory (DFT) and ab initio methods of different levels. This work has been focused in the elimination process of ethyl formate, as a good model of this kind of reaction. Different minima and the transition state have been found to characterize the thermal elimination reaction, and the activation energy has been obtained. In this study, we have used different methods to calculate the contribution of the correlation energy; different basis sets were also employed. This will allow us to perform an analysis of the influence of the applied method in the obtained results to study the thermal elimination. The lengthening of the bonds has also been analyzed to shown how concerted the thermal elimination is in the ethyl formate. Three minima have been found for the ethyl formate, two of them are very close and the third is higher in energy. In general, the DFT method with a B3LYP functional gives a less concerted reaction. A nonplanar transition state is predicted by most of the methods used.