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 a Ge 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. The reaction behavior observed with a Ge alloy containing 80 at.% Si resembles that generally obtained with pure Si: there are no easily identifiable steps between the initial Si–Ti sample and the final one, Si–TiSi2. With both the 50-50 and 80-20 Si–Ge alloys the formation of the C54 structure is preceded by that of the C49 structure (ZrSi2 type), as with pure Si. 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. Effects due to gas impurities on the path of the metal-substrate reaction have been analyzed. The resistivities of several pure and alloyed phases have been measured. Alloy scattering in the system TiSi2–TiGe2 is briefly discussed.