The semiconductor-rich region of the Si-Ge-Ti ternary isotherm at 900 degrees C was determined by metallography, x-ray diffraction, and electron microprobe analysis. The sample alloys were prepared by are-melting. These alloys were brought to equilibrium by annealing at 900 degrees C for 400 h. It was confirmed that at 900 degrees C TiSi2 and TiGe2 form a continuous solid solution Ti(Si1-yGey)(2) with the C54 crystal structure. It was also shown that, other than Ti(Si1-yGey)(2) and Si1-xGex, there is not any binary or ternary phase within the Si-Ge-TiGe2-TiSi2 trapezoid region; Between the Ti(Si1-yGey)(2) and Si1-xGex single-phase fields is the Ti(Si1-yGey)(2)-Si1-xGex two-phase region. The tie-lines for this two-phase region were determine The tie-lines tilt slightly toward the TiSi2 and Go corners. In other words, at equilibrium, the silicon to germanium atomic ratio is larger in Ti(Si1-yGey)(2) than in Si1-xGex (x > y). This tendency for tie-lines to tilt toward the TiSi2 and Ge corners had been proposed in the literature as the reason for the interesting microstructure evolution during the reactions between SiGe alloys and Ti. In addition, the possible diffusion paths for the reactions between SiGe alloys and Ti were discussed based on the obtained isotherm. Recognizing Si and Ge have higher mobilities in Ti(Si1-yGey)(2), it is predicted that for SiGe the extent of concentration change is large but occurs over a shorter distance, and for TiSi2, the extent of concentration change is small but occurs over a longer distance.