Abstract Displacement reactions of the type A + BX → AX + B have been investigated. Experiments have been carried out which affirm on the one hand the relation between the phase diagram of the system A-B-X and the layer sequence in a diffusion couple A/BX, and on the other hand permit to identify the rate-limiting step in the reaction process. To this end one of the reaction products has been applied as an initial layer on one of the couple halves before the heat treatment. In a sandwhich couple arrangement the effect of this initial layer on the morphology and thickness of the reaction zone has been directly compared with a true couple A/BX. The couples investigated up to now are Ni/Cu 2O at 1000°C, Fe/Ni 3S 2 at 460°C and Cu/Ni 3S 2 at 500°C.