An analysis of the influence of the operating conditions on the apparent first-order reaction for runs with constant heating rate in thermal decompositions has been carried out. A generalized treatment with dimensional variables has been considered. The influence of the heating rate, with or without heat transfer influence, in exothermic or endothermic decompositions has been studied. In addition, the influence of the reaction order and the residue at time infinity has also been studied. The simulation considers the decomposition kinetic law, the heat balance and the increase of the nominal temperature with time. Two models for the heat transfer have been considered : (a) in one model, the heat transfer coefficient does not include the variation of the sample mass, and (b) in the other model, the constant heat transfer coefficient includes the variation of the sample mass.