The effect of copper addition on the catalytic properties of silica-supported nickel catalysts for the reaction of CO hydrogenation in the temperature range of 200-500 degrees C has been investigated. Different effects, positive or negative, depending on the temperature and the copper content, are described and explained. At low temperature (230 degrees C), the addition of a low copper content prevents the loss of the active surface by sintering without inhibiting the rate of CO hydrogenation too much. At high temperatures (450 degrees C), high copper content is necessary to limit the accumulation of poisonous carbon products, but at the expense of CO conversion. On the basis of the various kinetic and morphologic effects of copper addition, an advanced description of the CO hydrogenation mechanism is also provided, assuming an active site formed by 2-3 adjacent Ni atoms, whatever the temperature or the copper content may be. (C) 2000 Academic Press.