A method for investigating the acid-base properties of solid surfaces using a small drop of unbuffered solution has been developed. When the aqueous solution comes into contact with a solid having ionizable functional groups on it, hydrogen ions are transferred between the solid and the solution via acid-base reactions. If the solution is a small drop of unbuffered one, it causes pH variation because of the change in the hydrogen ion concentration. The number of transferred hydrogen ions at a certain value of solution pH depends on the acid-base properties of the functional groups. Therefore, by measuring the pH variation of a small fixed-volume drop of unbuffered solution in contact with solids, it will be possible to determine the acid-base properties of solid surfaces.