Tips for scanning tunneling microscopy (STM) were modified with four kinds of self-assembled monolayers having functional groups with differing extents of hydrogen bond acidity or basicity. STM observation of CH3(CH2)(20)COO(CH2)(16)OH with the modified tips allowed the selective observation or discrimination of one from the other of the hydroxy and carboxylate moieties. With tips modified with basic 4-mercaptopyridine, the hydroxy groups were selectively observed as bright contrasts. Tips modified with 4-mercaptobenzenesulfonic acid, which contains an acidic sulfonyl group, gave bright contrasts for both the hydroxy and carboxylate moieties. However, tips modified with 4-mercaptobenzoic acid having a weaker acidic carboxy group exhibited bright contrasts only for the hydroxy groups. Unmodified tips did not reveal the position of the two functional groups. It was concluded that the change in image contrast occurs only at the position where hydrogen bond pairing is possible between tip and sample and requires strong hydrogen bond interaction between them. This suggests that the selective observation of oxygen-containing functional groups can be tailored by controlling the extent of the hydrogen bond acidity or basicity of the tip-modifying molecules. (C) 2003 Elsevier Science B.V. All rights reserved.