Na-23 relaxation NMR measurements and self-diffusion coefficients for sodium cations and p-sulfonatocalix[n]arenes (SCn) were obtained to confirm that monovalent inorganic cations are complexed by SC4. In the absence of added salts and at neutral pH. the cavity of p-sulfonatocalix[4]arene (SC4) fully binds an Na+ counterion. Our results provide evidence that when investigating the complexation of inorganic cations by SC4 a competitive binding scheme must be considered if more than one cation is present in solution. Moreover, it has been shown for the first time that SC6 and SC8 can also complex monovalent inorganic (sodium) cations.