Surfactant-stabilized NiB catalysts (ME-NiB) were prepared using the chemical reduction method in the ternary microemulsion system of water/cetyl-trimethyl-ammonium bromide (CTAB)/n-hexanol. The surfactant molecules could adsorb onto the surface of the formed particles, and acted as a protective agent to stabilize and restrict the growth of particles. The size of the nanoparticles was not fully determined by the size of the microemulsion droplets, which depended on the composition of the microemulsion system. Additionally, the particle sizes and the reactivities of the ME-NiB nanoparticles depended on the concentration of nickel salt, the amount and speed of addition of NaBH4 and the temperature. The ME-NiB samples were characterized by X-ray diffraction (XRD) as an amorphous structure and by transmission electron microscopy (TEM) as having particle size distributions in the range 3-8 nm, which was much smaller than that of NiB (20-50 nm) prepared by the typical chemical reduction method. The ultrafine catalyst of ME-NiB was markedly more active than NiB in the hydrogenation of carbonyl and olefinic groups. A good yield of citronellal/citronellol was obtained from the selective hydrogenation of citral at a low temperature of 30 degrees C over the ME-NiB catalyst. (c) 2006 Elsevier B.V. All rights reserved.