An integral isoconversional method provides a linear procedure to estimate the activation energy (E R ) at a given conversion (R) but only when an oversimplified approximation of the temperature integral (p(x), where x ) E/RT, R is the gas constant and T is the temperature) is used. Although the relative error in E R is several times less than the relative error in p(x) it happens to be substantial at x < 13. In the present study a nonlinear procedure to estimate E R has been developed. It shows very low relative errors in E R which are practically independent of x. The computations have been performed for model reactions as well as for the thermal decomposition of Cu 4 OCl 6 (triphenylphosphine oxide) 4 .