<p>Octreotide, a synthetic somatostatin analogue, is widely used in the control of acromegaly and GEP-NETs. As an octapeptide, its thermal stability profoundly impacts both formulation development and the quality control during storage. The thermal decomposition of octreotide acetate was investigated using thermogravimetry (TG), differential scanning calorimetry (DSC), thermogravimetry–mass spectrometry (TG-MS) and liquid chromatography–tandem mass spectrometry (LC–MS/MS). Under a nitrogen atmosphere, decomposition occurred in three stages, while four stages were observed under air, with the latter showing more complete decomposition. Activation energies for each stage were determined using the Friedman method, and the corresponding reaction model was successfully applied for lifetime prediction of octreotide acetate.TG-MS identified key evolved gases, including acetic acid, H₂S, SO₂, toluene, CO₂, and NO. LC–MS/MS revealed a two-step sulfur-release mechanism involving desulfurization of the disulfide bond, explaining two distinct sulfur-containing gas peaks. These findings provide critical insights into the thermal stability and degradation mechanisms of octreotide acetate, aiding in optimizing storage conditions and developing stable formulations. This study highlights the importance of thermal analysis and mass spectrometry in understanding peptide stability and guiding pharmaceutical development.</p> Graphical abstract <p></p>