The pH-dependent amount of aminopyrene adsorbed at the water surface was measured using a laser two-photon ionization technique. The distribution coefficients of neutral and cationic aminopyrene between the surface and bulk of liquid water were determined to be 9.6 x 10(-6) m and 2.5 x 10(-7) m, respectively. The acid-base equilibrium constant, pK(a), of aminopyrene on the water surface was determined to be 2.0, while that of the same molecule in bulk water was 3.6 +/- 0.1. These results show that neutral aminopyrene tends to stay on the water surface, resulting in equilibrium shifts toward the neutral form. It is demonstrated that laser two-photon ionization is a powerful method to analyze the equilibria of a small amount of molecules, 0.17pmol/cm(2), adsorbed at the water surface.