A new method has been proposed to determine the equilibrium between the bulk and the surface by directly measuring surface concentrations using laser two-photon ionization. This method has been applied to pyrenebutyric acid. The surface concentrations depended on the pH of the solution and were analyzed on the basis of two equilibrium constants and two distribution coefficients. Most pyrenebutyric acid stays on the surface at pH = 2.2. The equilibrium constant, pK(a), of pyrenebutyric acid on the water surface was determined to be 7.8(5) +/- 0.1(3), and this value is shifted to higher value than that in the bulk (4.76). The distribution coefficient of the neutral pyrenebutyric acid was determined as (5.9 +/- 2.8) x 10(-2) m, and that of pyrenebutyric anion as (4.8(2) +/- 0.1) x 10(-5) m. The ratio of the distribution coefficient of the neutral pyrenebutyric acid to that of pyrenebutyric anion was determined to be (1.2 +/- 0.6) x 10(3). These findings indicate that the equilibrium shifts toward the neutral form on the water surface. Laser two-photon ionization was found to be a sensitive and powerful technique to analyze equilibrium on the surface and that between the surface and the bulk.