[en] Two-nucleon transfer reactions induced by heavy ions are analyzed on the basis of the one-step (simultaneous) and two-step (successive) transfer processes. The reactions considered here are ¹²C(¹⁸O, ¹⁶O)¹⁴C, ⁴⁸Ca(¹⁸O, ¹⁶O)⁵⁰Ca and ⁴⁸Ca(¹⁶O, ¹⁴C)⁵⁰Ti. Finite-range calculations with full recoil are performed for both processes. The results of the calculations show that the two processes are of roughly equal importance and both are needed to reproduce the experimental angular distributions as well as the absolute magnitude of the cross sections. The dependence of the cross section on the choice of the structure wave functions and the relative importance of the intermediate states in the two-step process are also discussed. (Auth.)