A microinterface for ElectroSpray Mass Spectrometry (ESI-MS), acting as an electrochemical cell, was developed for the on-chip electrochemical tagging of cysteines in peptides and proteins during electrospray ionisation which occurs via a 1,4-Michael addition. Benzoquinones are electrogenerated from the corresponding hydroquinones to react specifically with cysteine residues and the production of the resulting adducts is followed by MS. The electrotagging efficiency was tested on L-cysteine using several hydroquinones bearing either electrodonating or withdrawing groups. The reaction kinetics was determined by electrochemical techniques such as Cyclic Voltammetry (CV) and was checked by MS experiments. In both experiments, carboxymethylhydroquinone was found to be the best tagging reagent. The electrochemical tagging was successfully applied to the identification of target proteins through the labelling of peptides coming from their proteolytic digestion. The information on the number of cysteine residues in proteolytic peptides was found to enhance the protein identification confidence.