When the equilibrium state of a reaction depends on the temperature, isoconversion methods for calculating activation energies are no longer valid. In these cases (apparent) activation energies obtained from isoconversion methods vary with temperature and fraction transformed, even if the true activation energy of the thermally activated process is constant. Expressions that describe the influences of the reaction parameters and the equilibrium state on the apparent activation energy are derived. A new method of extrapolating apparent activation energies is introduced that allows the determination of the true activation energy. Verification of this extrapolation method using precipitation in an Al-1 at% Si alloy shows a good correspondence between experiment and calculations. For the Al-1 at% Si alloy the apparent activation energy can deviate by 30% from the true activation energy of the reaction.