The fulvic acid molecular weight fraction studied has two general types of carboxyl groups, one of which is ortho to phenolic —OH groups. The number of each type per gram of fulvic acid has been measured. The acid ionization equilibrium of each type has been calculated as a function of its degree of ionization in 0.1 mKCl at 25.0 °C. In both cases the acid strength decreases with increasing degree of ionization. The sample used shows the potentiometric titration behavior expected of a low molecular weight polyelectrolyte.