Titanium reacts with pure Ge in two different ways. At low temperatures one observes the formation of Ti6Ge5 with some characteristics typical of diffusion-controlled reaction. Upon completion of this first stage Ti6Ge5 reacts with remaining Ge to form TiGe2, isomorphous with C54 TiSi2, in a process which is clearly controlled by nucleation. The same observations apply to reactions with an alloy containing 25 at% Si. With an alloy containing 50 at% Si the two stages become merged, so that while remaining identifiable, they are much less distinct than with the previous conditions. In the case of the 50-50 alloy, careful comparison of data obtained by backscattering, X-ray diffraction and resistance measurements reveals that in the formation of the mixed disilicide solid solution the formation of the C54 structure is preceded by that of the C49 structure as with pure Si. The reaction behavior observed with an alloy containing 80 at% Si resembles that generally obtained with pure Si: there are no easily identifiable steps between the initial SiTi sample and the final one, SiTiSi2. The gradual merging of the diffusion-controlled reaction and that controlled by nucleation as the concentration of Si in the substrate increases implies that nucleation plays a significant role in the formation of TiSi2, even if that role cannot be easily isolated. Values of the resistivity for both Ti6Ge5 and TiGe2 were obtained. © 1989.