Tailored cobalt-manganese mixed oxide nanoparticles are obtained from ionotropic gels of alginate, a seaweed-derived carboxylic-functionalized polymer. (Co,Mn)(3)O-4 phases are formed by low-temperature calcination of the alginate aerogels. Cubic spinel or tetragonal hausmannite structures are formed if, respectively, cobalt or manganese is the main cation. Stable nano-sized mixed oxides are formed in the whole range of composition of the solid solution by calcination of the aerogel precursors at 450 degrees C. The manganese-rich oxides are more dispersed than the cobalt-rich oxides, with surface areas higher than 80 m(2) g(-1). The organization of the oxide nanocrystals in arrays reminiscent of the fibrillar structure of the parent gel prevents them from packing and confers them peculiar magnetic properties in terms of coercivity and magnetoresistance. (C) 2012 Elsevier B. V. All rights reserved.