An approximate ternary phase diagram for the Si-Ge-B system has been established. Since neither germanium boride nor ternary phase exists in the system. Tie-Lines are found to connect silicon borides, i.e., SiB3, SiB6, and SiB14, to SixGe1-x alloys. While SiB3 is in equilibrium with SixGe1-x alloys with Si contents from 100 atom % down to 47 atom %, SiB6 establishes equilibrium with almost the rest of the composition domain ranging from 47 atom % Si down to 0.4 atom % Si. Calculations of the solid solubility of B in SixGe1-x, based on thermochemical data available in the literature, show that the solid solubility should decrease gradually from the value in Si to that in Ge. This is so because of the small, gradual variation in the thermochemical data (i.e., Gibbs energy values) when changing from one boride to the next. Experimental results show that the solid solubility does decrease gradually from about 7 X 10(19) atoms/cm(3) in Si to 1.6 X 10(19) atoms/cm(3) in Si0.25Ge0.75. However, it drops sharply to 6 X 10(17) atoms/cm(3) in Ge. Concentration-dependent interaction parameters have to be introduced in order to account for the rapid variation of the solid solubility of B in SixGe1-x with compositions close to Ge. The introduction of concentration-dependent interaction parameters is supported by the fact that while carbon borides and silicon borides are stable compounds, the gradually increased metallic character in the IVB elements from C to Pb prevents the formation of borides of Ge, Sn, and Pb.