A mixture of sodium phenylalanine and ethyl acetoacetate in aqueous solution underwent enamine formation as determined by nuclear magnetic resonance- and ultraviolet-spectroscopic measurements. An equilibrium state was observed in the aqueous solution between the enamine and the starting materials. Although the observed equilibrium constant, K_obs, showed dependency on the pH value of the solution, the equilibrium constant, K, was not influenced by the pH value. The effect of pH values on K_obs was related to the pK_a value of phenylalanine, suggesting that only the unionized form of the amino moiety of phenylalanine could take part in the enamine formation.