One of the intermediates of finasteride was oxidized by the reaction of benzeneseleninic (BS) anhydride in refluxing toluene for 16 h. Sometimes, the desired product was obtained, and sometimes, there was no yield or very low yield at the end of the reaction time. To obtain a better understanding of such variations, laboratory syntheses were conducted with commercial BS anhydride, pure BS anhydride, pure BS acid, and mixtures of anhydride and acid samples. During the reaction, the intermediate reaction masses were analyzed by high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) at predetermined times and correlated with differential scanning calorimetry (DSC) and X-ray powder diffraction (XRPD) samples. The DSC and XRPD data indicated that commercial BS anhydride samples did not always consist of the pure anhydride form. When the BS acid was present in the anhydride sample, the reaction rate was lower than that of the pure BS anhydride sample.