Rod-shaped nanostructures of cobalt oxalate dihydrate were synthesized at room temperature by the microemulsion (reverse micellar) route. These rods are highly uniform in length and can be modified with temperature (from similar to 6.5 mu m at 50 degrees C to similar to 2.5 mu m at 150 degrees C) while keeping the diameter nearly constant (200-250 nm). Thermal decomposition of these rods in a controlled atmosphere (air and H-2) leads to nanoparticles of Co3O4 and Co, respectively, while in a helium atmosphere a mixture of Co and Coo nanoparticles is obtained. Co3O4 nanoparticles (similar to 35 nm) were slightly agglomerated, while Co nanoparticles were monodispersed and highly uniform (similar to 25 nm). The oxalate rods and Co3O4 nanoparticles show an antiferromagnetic ordering at 54 and 35 K, respectively. (c) 2008 Elsevier Inc. All rights reserved.