A self-assembled monolayer of thiourea-based anion receptor was formed at a gold surface, and its affinity with monovalent inorganic anions (MeCO2-, H2PO4-, Cl-) in acetonitrile solution was examined by second harmonic generation spectroscopy. The estimated adsorption coefficients of anions at the monolayer reveal specific binding selectivity compared with the selectivity found in a bulk acetonitrile phase.