Co/AlN/SiO2 catalysts were prepared by the saturative chemisorption of cobalt(III) acetylacetonate (Co(acac)(3)). The support was bare silica or silica that had been modified with aluminum nitride (AlN) by repeated separate, saturated chemisorptions of trimethylaluminum and ammonia two or six times. Chemisorption of Co(acac)(3) occurred on all the supports up to a saturation ligand density of 2.7 acac nm(-2); the amount of bonded cobalt decreased from 2.1 to 1.5 at(Co) nm(-2) with increasing extent of AlN modification of the support. Ligand exchange reaction, releasing Hacac, occurred less on AlN-modified silica than on bare silica. This induced difference in the reduction behavior of the catalysts, and catalytic activity in gas-phase hydroformylation of ethene, was lower with Co/AlN/SiO2 than with Co/SiO2 catalyst.