Gibbsite crystallization kinetics have been measured in synthetic Bayer liquors in the presence of sodium gluconate, a known inhibitor of gibbsite growth and nucleation. The kinetics data have been fitted using a kinetics model (Kubota and Mullin, 1995) that assumes Langmuir adsorption of the impurity to the crystal surface and an effectiveness factor that allows the adsorbed impurity to block more of the growth surface than it physically covers. The crystallization kinetics and supporting evidence imply that the gluconate ion is not chemisorbed to the gibbsite surface. At 70°C, the Langmuir adsorption constant is evaluated as 770 ± 50 L mol– 1 and the effectiveness factor as 1.51 ± 0.05. From examination of electron micrographs, it is seen that crystal growth is inhibited on both the basal and prismatic planes, in contrast to what occurs with typical Bayer organics. Gluconate adsorbs preferentially to the “birth and spread” sites and this Langmuir adsorption constant is specific to the active growth sites. Hence, the measurement of an adsorption isotherm by measuring the impact upon the growth kinetics better addresses the question of how much of the growing surface is covered by an adsorbing organic.