This work reports the first in situ and real-time resolved spectroelectrochemical monitoring of a diazonium electrografting. Exploiting the 4-nitrobenzene diazonium salt reduction, we provide a spectral characterization of the adsorption kinetics and confirm the control of the layer growth by using dissolved atmospheric dioxygen as a redox mediator. Gravimetric data were used to rationalize the absorption results and confirm the accuracy of the coupled technique. By comparing the deposition on gold and carbon substrates, the spectroelectrochemical measurements highlighted the impact of the underlying material on the spectral behavior of the organic layer and suggested the existence of an unexpected interface continuum.