Molybdenum nitrogenase catalyses the reduction of N₂ to NH₃ at its cofactor, an [(R-homocitrate)MoFe₇S₉C] cluster synthesized via the formation of a [Fe₈S₉C] L-cluster prior to the insertion of molybdenum and homocitrate. We have previously identified a [Fe₈S₈C] L*-cluster, which is homologous to the core structure of the L-cluster but lacks the 'ninth sulfur' in the belt region. However, direct evidence and mechanistic details of the L*- to L-cluster conversion upon 'ninth sulfur' insertion remain elusive. Here we trace the 'ninth sulfur' insertion using SeO₃^2- and TeO₃^2- as 'labelled' SO₃^2-. Biochemical, electron paramagnetic resonance and X-ray absorption spectroscopy/extended X-ray absorption fine structure studies suggest a role of the 'ninth sulfur' in cluster transfer during cofactor biosynthesis while revealing the incorporation of Se^2-- and Te^2--like species into the L-cluster. Density functional theory calculations further point to a plausible mechanism involving in situ reduction of SO₃^2- to S^2-, thereby suggesting the utility of this reaction to label the catalytically important belt region for mechanistic investigations of nitrogenase.