In a previous paper, it was shown that the functional polysaccharide kappa -carrageenan acts as an efficient stabilizer to prevent the precipitation of iron oxides and hydroxides up to very high pH values (Jones, et al. Colloid Polym. Sci. 2000, 278, 491-501). Here, this process was investigated for its application to synthesize stable cobalt and nickel oxide particles. Nickel hydroxide nanoparticles were well stabilized for long periods, while the hydroxide of cobalt(II), although initially stable at pH 13, progressively showed some complex structure rearrangement, invoked by alkaline oxidation of the primary particles. The final product of this oxidation/self-assembly process are spherical, fluffy superstructures of cobalt(III) oxide platelets with an overall diameter of 12 mum, in coexistence with 65-75% of free kappa -carrageenan.