The applicability of the following isoconversional (model-free) incremental methods for determining the energy of activation of the heterogeneous processes under arbitrary temperature programs are discussed: advanced nonlinear isoconversional method (A-NL method), iterative incremental isoconversion method (IT-method) and incremental differential method (Incr-dif method). It is justified that when a process takes place in some arbitrary temperature programs, always applying the A-NL method considering that dependence T vs. t is linear even for narrow domains of the degree of conversion leads to erroneous activation energy values. A procedure for applying the A-NL method applicable to any temperature programs (modified A-NL method) has been suggested. This procedure has been verified for simulated sinusoidal modulated data, experimental data obtained by TG analysis of HDPE under quasi-isothermal conditions, and experimental TG data obtained in investigating thermal decomposition of LDPE under arbitrary temperature programs. For this purpose, the values of the activation energy obtained by the modified A-NL, IT and Incr-dif were compared.