Cyclic voltammetry, differential capacity and chronocoulometry have been employed to study pyrazine adsorption quantitatively at the Au(111)/aqueous solution interface. The adsorption isotherms, Gibbs energies of adsorption and the electrosorption valency for pyrazine adsorption at the Au(111) surface have been determined. The results indicate that pyrazine adsorption at gold has the character of weak chemisorption which involves either the interaction of pi orbitals of the aromatic ring with free electrons at the metal surface or a mixing of the n orbital at the nitrogen atom with d electronic states in the metal. Consequently the pyrazine molecule may assume either the pi-bonded flat or the N-bonded vertical surface coordinations. The flat orientation is stabilized at the negatively charged surface, and the vertical orientation is stabilized at the positively charged surface. A reorientational phase transition in the monolayer of adsorbed pyrazine was observed at the potential of zero charge.