Electrical double layer (EDL) formation at the electrode/electrolyte interface is an important interfacial phenomenon in energy storage using supercapacitors. In this work, electrosorption experiments and Grand Canonical Monte Carlo (GCMC) simulations are employed to better understand EDL formation in charged nanopores for mixtures of electrolytes. Experimental results indicate that electrosorption depends significantly on electrolyte properties. Competitive effects take place inside nanopores. Simulation results are in qualitative agreement with experimental data. For mixtures of electrolytes, the competitive effects of ion charge and size asymmetries determine the pore accessibility. Divalent counterions preferentially screen the surface charge due to their higher valence. On the other hand, small counterions present “size affinity” to access nanopores and approach closer to charged surfaces. Electrosorption selectivity of the nanopores for counterions is defined by a counterbalance between minimization of potential energy and size exclusion effects. These two competitive effects result in electrosorption selectivity of small monovalent over large divalent ions, depending on the surface charge density. As the charge density increases, the pore selectivity for large divalent counterions increases, reaches a maximum, and then turns into higher selectivity for small monovalent counterions. The findings of this work have significant implications in developing novel supercapacitors for energy storage.