The reduction of nickel/silica catalysts was studied in parallel by temperature programmed reduction (TPR) and X-ray diffraction (XRD) methods. The calcined materials contain nickel in the form of nickel oxide and nickel hydrosilicates and a part of the oxide particles is covered with the hydrosilicate species. This oxide is reduced in the same way as unsupported nickel oxide in the absence of water, and in a similar way to nickel hydrosilicates in the presence of water.