The initial rise method is one of the most reliable methods for evaluating the activation energy (E) of a single glow peak. However, this method gives erroneous values of activation energy in cases where the thermal quenching effect is present during the initial rise measurements. It is well established that the thermal quenching effect is dominant in the thermoluminescence of Al2O3:C. The experimental glow-curve shape is highly disturbed by thermal quenching, so it cannot give reliable information about either its structure or the values of the trapping parameters, i.e. the activation energy, the frequency factor and the order of kinetics. The presence of a strong thermal quenching effect in Al2O3:C makes this material ideal to demonstrate the influence of this effect on the resulting values of activation energy as well as to examine suggested correction methods.