This study investigates the effect of cation species on the onset of electrolyte hydrolysis in hybrid supercapaci-tors with aqueous electrolytes using isothermal operando calorimetry. The cells consisted of a positive a-MnO2 cryptomelane electrode and a negative activated carbon (AC) electrode with either 0.5 M K2SO4 or 0.5 M Cs2SO4 aqueous electrolytes. They were characterized using cyclic voltammetry and galvanostatic cycling. In addition, the instantaneous heat generation rate at each electrode was measured using a custom isothermal operando calorimeter. Heat generation associated with resistive losses (Joule heating) and reversible ion adsorp-tion/desorption was clearly identified. For larger potential windows, an endothermic dip, attributed to the on-set of hydrolysis, was observed at the positive a-MnO2 electrode where K+ and Cs+ ions engaged in fast surface redox reactions. Interestingly, this endothermic dip appeared at 1.8 V and 2.0 V for K2SO4 and Cs2SO4 aqueous electrolytes, respectively. The difference in the stable operating potential window was attributed to thinner solvation shell around Cs+ cation than for K+ thus reducing the amount of water present near the electrodes as ions partially shed their solvation shells during adsorption. The early onset of hydrolysis could be observed by isothermal operando calorimetry before it could be observed with conventional electrochemical methods.