Self-assembled monolayers (SAMs) of 2-[(6-mercaptohexyl)oxy]methyl-12-crown-4 (1) and 2-[(6-mercaptohexyl)oxy]methyl-15-crown-5 (2) on gold are able to bind cations reversibly from aqueous solutions. This binding of the electrochemically inactive cations is monitored by cyclic voltammetry and impedance spectroscopy using the redox couple Ru(NH3)62+/3+ as "reporter ion". Gold electrodes modified with 1 detect Na+ even in the presence of a 100-fold excess of K+; SAMs of 2 are K+ selective. As a result of the high surface density of crown ethers, sandwich complexes are formed. Reduced surface concentrations of receptors in mixed monolayers of 2 with heptanethiol prevent the sandwich complexation and decreases the K+/Na+ selectivity of the SAM.