This project was started when an unknown peak was being detected in the organic volatile impurity (OVI) analysis (using the head-space vapor sampling procedure) of bortezomib (BZB) active pharmaceutical ingredient (API), in pharmaceutical companies. During the OVI analysis of the API of BZB, a huge-area peak with an unknown source appeared in the chromatograms of the gas chromatography with flame ionization detector (GC-FID). The data prepared by GC-MAS revealed that the considered huge peak was 3-methylbutanal (3MBut). But, investigating the synthesis procedures showed that during all the synthesis steps, 3MBut or any other solvent containing this impurity was not being applied. Thus, we had concluded that there is a possibility for emergence of this aldehyde from bortezomib itself. To find out which part of bortezomib might turn into 3MBut, we began to investigate all its molecular structure, and hypothesized that the α-amino-boronic acid part of the molecule turned into 3MBut. The experimental analysis and theoretical quantum chemical calculations confirmed that the α-amino-boronic acid center of bortezomib molecule undergoes a rare and unexpected aerobic oxidation by O2 molecule, even in catalyst and solvent-free conditions. The result of this project not only might make clear the passive source of the 3MBut peak of the OVI of bortezomib, but also, it would suggest to store this API in an inert oxygen-free atmosphere to improve the long-term and accelerated (thermal) stability of this very expensive anti-cancer drug.