The adsorption of ions onto solid surfaces may be via inner-sphere or outer-sphere mechanisms. When inner-sphere, the process also involves the loss of water molecules from the ion's hydration sphere. In general, ions are strongly held by inner-sphere adsorption and weakly held by outer-sphere adsorption. Accordingly, any property that enhances the stability of an adsorbed inner-sphere ion will also increase the overall strength of the adsorption mechanism. We hypothesize that the space limitations inside pore channels will effectively stabilize inner-sphere adsorption processes. The adsorption of Ni²⁺ and Na⁺ cations on various zeolites were monitored as a function of pH. The zeolites used varied in their mean pore radius dimensions. We found that when the pore radius is large, Ni²⁺ adsorption was much stronger than Na⁺ adsorption. This is the same as what one normally finds for adsorption on flat, unconstrained, external surfaces. However, when the mean pore radius is intermediate in size, we found no change in the adsorption of Ni²⁺, but the adsorption of Na⁺ became far stronger than the adsorption of Ni²⁺. When the mean pore radius was small in size, we found that both the Ni²⁺ and the Na⁺ cations adsorbed very strongly. The adsorption between Ni²⁺ and Na⁺ on zeolite was also found to be competitive in nature. We propose that these observations are a direct result of each cation's radius of hydration relative to the radius of the nanopore structure.