Grand canonical Monte Carlo (GCMC) simulations were performed to study the fundamental aspects of electrosorption selectivity of electrolyte mixtures inside nanopores. Simulation results indicated that exclusion of coions is associated with the population of different counterions involved in the electrical double layer (EDL) formation. In the case of electrolyte mixtures, the competitive effects of asymmetries of ion charge and size can determine the pore accessibility. Divalent counterions have the energetic advantage of preferentially screening the surface charge. On the other hand, small counterions can access the pores and approach closer to the charged surfaces. Electrosorption selectivity of small monovalent versus large divalent ions depends on the magnitude of surface charge. With increasing surface charge density, the preferential selectivity of the pores for large divalent counterions is transferred to that of small monovalent counterions. Such behavior occurs because of the competitive effects of electrostatic potential and size exclusion. Furthermore, the ion distribution inside the pores provides an insight into the mechanisms behind these competitive effects. The population of ions present in the double-layer region of the pore not only affects the screening of surface charge but also dominates the pore occupancy (i.e., accumulation of different counterions and exclusion of coions). Therefore, electrosorption selectivity may be achieved via manipulation of the EDL formation inside the pores to separate ions from electrolyte mixtures. The results of this work have several significant implications in ion separations and energy storage.