Abstract: The paper reported fast bombardment mass spectra of Mo_3 S_7 (S_2P(OEt)_2)_3 C1 and obtained satistactory mass spectra. The spectra exhibit six very broad peak groups over a wide range of m/z values for each ion species due to the polyisotopic character of Mo but also ~(13)C and ~(24)S. We also obtained satisfactory mass spectra when using conventional electron impact ionization mass spectra for Mo_3 S_4 (RCO_2) (S_2P (OEt)_2)_3 Py (R=H, C_2H_5, Py=NC_5H_5). It is found that two types of broad peak groups which come from Mo_3 OS_3(RCO_2)(S_2P(OEt_2)_3Py due to theμ_3—S atom exchanged with μ_3—O atom in Mo_3S_4(RCO_2)(S_2P(OEt)_2)_3Py and according to normal unimolecular ion decomposition of Mo_3S_4 (RCO_2) (S_2P (OEt)_2)_3Py. Finally we discussed the problem about the 'oxygen-source' in the experiment of mass spectrometry.