Ge1xSnx nanorods (NRs) with a nominal Sn content of 28% have been prepared by a modified microwave-based approach at very low temperature (140 C) with Sn as growth promoter. The observation of a Sn-enriched region at the nucleation site of NRs and the presence of the low-temperature -Sn phase even at elevated temperatures support a template-assisted formation mechanism. The behavior of two distinct Ge1xSnx compositions with a high Sn content of 17% and 28% upon thermal treatment has been studied and reveals segregation events occurring at elevated temperatures, but also demonstrates the temperature window of thermal stability. In situ transmission electron microscopy investigations revealed a diffusion of metallic Sn clusters through the Ge1xSnx NRs at temperatures where the material composition changes drastically. These results are important for the explanation of distinct composition changes in Ge1xSnx and the observation of solid diffusion combined with dissolution and redeposition of Ge1ySny (x > y) exhibiting a reduced Sn content. Absence of metallic Sn results in increased temperature stability by 70 C for Ge0.72Sn0.28 NRs and 60 C for Ge0.83Sn0.17 nanowires (NWs). In addition, a composition-dependent direct bandgap of the Ge1xSnx NRs and NWs with different composition is illustrated using Tauc plots.