The locations and widths of the transition zones for the Landau-Zener and the Demkov model are calculated in a straightforward way. A single transition near a diabatic curve crossing (Landau-Zener model) is found to be related to two transition zones located on either side of the crossing. A single transition between parallel diabatic states induced by an exponentially varying coupling potential (Demkov model) is found to be related to a single transition zone. For low collision velocities the transition zones are determined by the adiabatic potential energy difference and are independent of the collision velocity for both systems. For high velocities the zones are found to depend on the velocity. Finally, the validity of the Landau-Zener model is investigated for non-constant coupling potentials.