alpha-Cobalt hydroxide is precipitated by soda from its sulphate salt in a double-jet standard stirred reactor, and its transformation into its stable phase beta is characterized by recording the pH of the solution. The alpha precipitate is amorphous, whereas, from XRD patterns, the beta precipitate is crystalline; its SEM micrographs display thin hexagonal platelets. Higher sulphate concentration in the reactor increases their size, but reduces the transformation kinetics only when added before the injection of reagents. The transformation is found to be controlled by the dissolution of the alpha precipitate: the sulphate ions both reduce the dissolution of the micronic agglomerates of alpha precipitate, as they are probably adsorbed on their internal surface, and lead to wider beta platelets, because of their specific adsorption on their two lateral faces.