Approximate models for diffusion-reaction having been presented so far, described processes with a single balance equation. In the present work, the approximate models for a single reaction with thermal effect or a single two-component reaction in a catalyst pellet have been derived and discussed. The formulas are typical long-time approximations of the process. Accuracy of the models at a steady-state is good, especially in the range of small and intermediate values of the Thiele modulus.