Ge1-xSnx nanorods (NRs) with a nominal Sn content of 28% have been prepared by a modified microwave-based approach at very low temperature (140 degrees 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 alpha-Sn phase even at elevated temperatures support a template-assisted formation mechanism. The behavior of two distinct Ge1-xSnx 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 Ge1-xSnx NRs at temperatures where the material composition changes drastically. These results are important for the explanation of distinct composition changes in Ge1-xSnx and the observation of solid diffusion combined with dissolution and redeposition of Ge1-ySny (x > y) exhibiting a reduced Sn content. Absence of metallic Sn results in increased temperature stability by similar to 70 degrees C for Ge0.72Sn0.28 NRs and similar to 60 degrees C for Ge0.83Sn0.17 nanowires (NWs). In addition, a composition-dependent direct bandgap of the Ge1-xSnx NRs and NWs with different composition is illustrated using Tauc plots.