We measured 209Bi(n, 4n) cross sections at neutron energies En = 29.8 ± 1.8 MeV and En = 34.8 ± 1.8 MeV. Bismuth oxide samples were irradiated with the neutrons produced by impinging 30, 35 and 40 MeV proton beams on a 1.05 cm thick beryllium target, where the proton beams were from the MC-50 Cyclotron of Korea Institute of Radiological Medical Sciences (KIRAMS). The neutron flux for each proton beam energy Ep, ΦEp(En), has a broad spectrum with respect to En. By taking the difference in the neutron fluxes, the difference spectra, Φ40(En) −Φ35(En) and Φ35(En) −Φ30(En), are obatined and found to be peaked at En = 29.8 and 34.8 MeV, respectively, with a width of about 3.6 MeV. By making use of this observation and employing the TENDL-2009 library we could extract the 209Bi(n, 4n)206Bi cross sections at the aforementioned neutron energies.