The reduction of NO by CO over silica was studied in the temperature range from 873 to 1273 K under an initial gas (NO, CO) pressure of 395 Pa. Reduction was carried out by a volumetric technique with a mass spectrometer for the analysis of the gaseous phase. The catalytic activity of silica is enhanced by its pretreatment in air or in vacuum in the temperature interval 873-1273 K and as a result the rate of reduction of NO is increased by a factor of >3. In contrast, the rate of reduction of NO is not enhanced by pretreatment of the silica with water vapour at 973 K. The catalytic activity of silica seems to depend on the presence of a particular superficial structure occurring during heat treatment.