Early stages of precipitation in an Al-0.5 at % Ge-0.5 at % Si alloy during aging at 160degreesC have been examined using a combination of transmission electron microscopy (TEM) and X-ray diffraction (XRD). Within 1/2 h of aging, TEM revealed the formation of fine, equiaxed GeSi precipitates having the diamond cubic structure. Further aging causes nucleation and growth of such precipitates and formation of GeSi precipitates having two other morphologies i.e. triangular plate and <100>(A1), lath. XRD revealed that the lattice parameter of the diamond cubic GeSi precipitates increases gradually with aging time up to 24 h i.e. the peak aged condition. These results are discussed in terms of critical requirement of vacancies for the nucleation and growth of GeSi precipitates, and increasing. volume misfit strain between precipitate and matrix A1 with increasing lattice parameter of the precipitates.