Rate equilibrium dialysis was used to measure the binding of 2,3-diphosphoglycerate (DPG) to human oxy- and deoxyhemoglobin A0 over the range pH 5-9, at 21.5 degrees C. This approach yielded an accurate, precise, and self-consistent set of model-independent association constants. These data were successfully fitted to a thermodynamic model which is functionally similar to a Hill equation. The isotherms generated by this fitting procedure appear to intersect at low pH and converge at high pH. This apparent convergence at high pH is consistent with results obtained by oxygen equilibria studies performed under conditions of saturating DPG. These calculated isotherms were used to determine the enhancement of the Bohr effect as a function of pH. These results are consistent with data obtained by pH stat measurements by other investigators. This paper presents the first in a series of studies that will provide a systematic characterization of the interaction between hemoglobin and DPG.