[en] The relevance of one- and two-dimensional approaches to two-ion mode conversion in Tokamaks is discussed, in view of experimental observations in Tokamaks using laser or microwave scattering techniques. The problems of evaluation of the condition for fast wave resonance in the two-dimensional approach is analysed. Simplified ways to find the mode conversion surfaces when they exist are presented. Also for large Tokamaks, it is possible to obtain two-ion mode conversion for realistic antenna spectra, provided ion species concentration and frequency are chosen such that the surface S = 0 intersects the plasma midplane just outside of the magnetic axis. (author)