This paper reports a study on the formation mechanism of nanoparticles (NPs) composed of bismuth, antimony and tellurium for thermoelectric materials using a modified polyol synthetic route. Our one-pot synthesis technique has proven highly versatile in creating a wide range of different anisotropic NPs such as nanowires (NWs), nanodiscs (NDs), nanoribbons and nanospines (NDs studded on NWs) simply by modifying the capping species or elemental precursor feeding ratio used in the synthesis. However, an independent control of morphology and composition is still hugely challenging and the facile synthesis of (Bi,Sb)₂Te₃ solid solution NPs is not a trivial task, reflecting the complex nature of this multicomponent system. To achieve this goal, it is imperative to understand the formation mechanism based on a systematic investigation of mono- and binary elemental NP systems. Our study clearly shows the different actions of oleylamine (OAM) and decanethiol (DT) capping ligands in our synthesis reaction. In the case of DT capping system, Te NDs are first formed, and then, Bi and Sb are separately incorporated into the Te ND structure viacatalytic decomposition of Bi-DT and Sb-DT complexes on the Te ND surfaces. Therefore, the resulting NPs are phase segregated into Te, Bi₂Te₃ and Sb₂Te₃. On the other hand, in the case of the OAM capping system, Te NWs and Bi-Sb solid solution NPs are formed separately, and then, parts of Te NWs are transformed into (Bi,Sb)₂Te₃ phase via oriented attachment of Bi-Sb NPs and Te NWs. These findings are crucially important towards the one-pot synthesis of uniform (Bi,Sb)₂Te₃ nanobuilding blocks with controllable characteristics for highly efficient thermoelectric materials.