The formation of hydrotalcite in mixed suspensions of magnesium hydroxide and aluminum hydroxycarbonate was found to be influenced by factors affecting heterocoagulation of the separate particles. A model of the system employing large particles of magnesium hydroxide exposed to an aluminum hydroxycarbonate suspension demonstrated rapid hydrotalcite formation when pH conditions were adjusted to prohibit heterocoagulation. Scanning electron micrographs of the particles indicated that nucleation and growth of the mixed crystal was occurring directly on the magnesium hydroxide surface. The appearance of hydrotalcite was delayed in mixed suspensions wherein heterocoagulation resulted in an occlusive coating of aluminum hydroxycarbonate on the magnesium hydroxide. Formation of hydrotalcite occurred sooner in systems having higher initial pH values presumably due to earlier exposure of magnesium hydroxide surfaces resulting from more rapid aging of the aluminum hydroxycarbonate coating. Mixed suspensions of magnesium hydroxide and other aluminum-containing hydroxides, wherein an occlusive coating did not form on the magnesium hydroxide particles, demonstrated rapid hydrotalcite formation.