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During crystallization of biological macromolecules by the vapor diffusion method, with ammonium sulfate as the precipitant, pH variations were shown to occur in the crystallization droplet [Mikol, Rodeau & Giegé, (1989). J. Appl. Cryst. 22, 155–161]. Using pH-sensitive electrodes, the kinetics of these pH changes have now been experimentally studied as a function of the volume (10 to 50 μl) and initial pH (6.0 or 8.0) of the droplet, with a reservoir at pH 7.0, at 278, 286 and 293 K. A theoretical analysis is given and a model for estimating these kinetics under given crystallization conditions is proposed, illustrated by an application to the time course of concanavalin A supersaturation.