Capillary electrophoresis (CE) coupled with electrospray ionization (ESI) mass spectrometry (MS) is a suitable technique for the analysis of intact proteins. The main configuration to realize this coupling is the sheath liquid interface, which is characterized by the addition of a make-up liquid providing the electric contact as well as the appropriate flow and solvent composition for optimal ionization and evaporation. One main advantage of this interface is that the composition of the sheath liquid can be tuned to modify the ionization without affecting CE selectivity and efficiency. In the case of protein ionization, this feature is particularly interesting to modulate their charge-state distribution (CSD), while keeping the separation performance unchanged. In this context, the current work evaluated the effect on proteins' CSD of adding supercharging molecules to the sheath liquid. Several supercharging reagents were tested with different background electrolyte (BGE) and their impact was estimated for three model proteins (i.e., human insulin, human growth hormone, hemoglobin A0) exhibiting various properties in terms of ionization, conformation, and flexibility. Their influence on the global sensitivity for each protein was also assessed. Among the supercharging reagents tested, m-NBA and sulfolane led to supercharging effect whose magnitude depended on the proteins as well of the BGE pH. The sensitivity and separation performance remained globally unchanged for each protein and supercharging additive, while sulfolane led in some cases to a sensitivity improvement.