It is shown that adsorption of nucleic acid molecules and their monomeric components, i. e., nitrogenous bases, from aqueous solutions results in the formation of an insulating layer on the germanium surface. Comparatively small values of the insulator charge and the surface-state density point to promising applications of nucleotides for both the formation of germanium-based insulator-semiconductor structures with nanoscale insulating layers and low surface-state densities at the phase interface, and for germanium surface passivation. Changes in the electronic properties of the space-charge region of germanium during nucleotide adsorption on its surface can be used as a method for determining the nucleotide molecule concentration in aqueous solutions.