The interaction of adsorbed phosphate anions with alkali metal cations at the Ag\aqueous solution interface has been investigated by surface-enhanced Raman spectroscopy (SERS). Formation of ion pairs at the interface was evident from the cation-induced perturbations in the SER spectra of anions. The frequency of the external vibration, silver-oxygen (Ag-O '), was not sensitive to the nature of cation, while the relative intensity of this mode was cation-dependent and was explored as a sensitive probe for the monitoring of coadsorption of ions at the interface. From the internal phosphate vibrations, both asymmetric modes, delta (as)(PO) and v(as)(PO), were found to be the most sensitive to the nature of the cation. At a relatively positive potential 0.00 V vs. Ag /AgCl/ the spectral parameters for the Cs+ and K+ cations were very similar indicating the same bonding type with anions. A more inhomogeneous chemical environment for the phosphate oxygen atoms was detected in the case of Na+ and Li+ cations. An increase in v(as)(PO) frequency by ca. 10 cm (1) was the characteristic spectral signature for the interaction of phosphates with Li+. The formation of water-shared ion pairs at the interface was suggested based on the absence of splitting in the v(as)(PO) mode and the previously observed frequency sensitivity of this band to solvent H2O substitution by D2O. At negative potential (-0.80 V), a stabilization effect of Cs+ on the phosphate adlayer was detected based on the twofold increase in intensity of the v(Ag-O ') mode compared with Li+. Splitting of the v(as)(PO) mode suggested the contact interaction of anions with specifically adsorbed Cs+ cations.