The reciprocal influence of the phenylsubstituents and diazonium groups allows to monitor the diazonium reactivity and to electrochemically detect the grafting reaction. Extended understanding concerning the grafting of para substituted tetrafluoroborate aryl diazonium salts p-(R–Ph–N₂⁺, BF₄⁻) was obtained by studying comparatively four compounds (R=NO₂, NEt₂, NHPh, NPh₂) by electrochemistry. For R=NEt₂, the grafted molecules showed no reversible electroactivity whereas for the aminophenyl substituents, the first oxidation process induced chemical modification of the deposited layers before being totally reversible. The compound with electron withdrawing group (NO₂) was the only one able to create spontaneous covalent bounding with the glassy carbon electrode (GCE). We observed that the substituent directly acts upon the diazonium reactivity. This effect can be directly monitored through the potential onset value of the diazonium reduction. In addition, the elimination of the diazonium group during electrografting induces a cathodic shift of the electroactivity of the aminodiphenyl and aminotriphenyl groups covalently attached onto the carbon electrode surface. The shift between these electrochemical values may be considered as a signature of the grafting reaction.