The application of radiotracer method in the in situ studies of adsorption on semiconductor electrodes has been described. Using this technique the adsorption of HCOOH on n-Si(100) electrode has been studied and the surface concentration of the adsorbate determined. The adsorption takes place in the potential range of -0.5-1.0V. It has been found that adsorption of HCOOH is an irreversible process. The dependence of the surface concentration of adsorbed species on the bulk concentration of HCOOH has been determined, too. The epm and eps values at the surface concentration close to saturation are equal to 0.4 and 0.9, respectively. It has been found that adsorbed species are arranged in two layers. It is proposed that the first layer is formed by COOH radicals and the second layer is formed by HCOOH molecules. Both layers are most likely linked by hydrogen bonds.