The thermodynamic stabilities for the intermetallic compounds in the Ge-Ti system were studied using metallography, powder x-ray diffraction, and electron probe microanalysis. It was found that TiGe is not stable at 700 and 900 degrees C, and Ti3Ge is not stable at 700 and 800 degrees C. The other three compounds TiGe2, Ti6Ge5, and Ti5Ge3 are stable. A diffusion couple study between Ti and Ge was also performed at 800 degrees C for 105 h. It was found that only Ti6Ge5 and TiGe2 formed, but Ti5Ge3 did not. From the diffusion couple, it was found that Ti6Ge5 is not a line compound, but has an estimated homogeneity range from 42 at.% to 47 at.% Ge. (C) 1999 Elsevier Science S.A. All rights reserved.