CoNiB and polymer polyvinylpyrrolidone (PVP)-stabilized CoNiB (PVP-CoNiB) catalysts were prepared by chemical reduction with NaBIH4. They were characterized and examined for their catalysis in the hydrogenation of unsaturated aldehydes. CoNiB and PVP-CoNiB catalysts were characterized by X-ray diffraction as amorphous structures and by transmission electron microscopy as having particle sizes in the range 3.5-7 nm - smaller than those of NiB (7-15 nm) and CoB (5-10 nm). CoNiB catalysts were significantly more active than NiB and CoB in the hydrogenations of furfural, crotonaldehyde and citral, and the PVP-CoNiB catalysts were significantly more active than CoNiB. The catalytic properties of CoNiB and PVP-CoNiB catalysts during the selective hydrogenations of crotonaldehyde and citral were similar to those of NiB, but different from those of CoB. The conjugated C = C bonds in crotonaldehyde and citral were preferentially reduced to form butyraldehyde and citronellal. PVP-CoNiB could hydrogenate citral to citronellal at a low reaction temperature of 30 degrees C. (C) 2008 Elsevier B.V. All rights reserved.