Gasil-I (a mesoporous silica) and TK800 (a non-porous pyrogenic silica) were investigated in the early 1970s as standard reference materials. Since then the specific surface areas of both silicas have decreased to similar to 85% of their initial values, suggesting that the surface character and the ageing mechanism may be the same for both. Comparisons of the shapes of nitrogen-adsorption isotherms, confirmed by comparisons of the shape ratios for Gasil-I and TK800, indicate that Gasil-I has greater microbore character and a higher absorption at p/p(0) > 0.5 than TK800 and that the isotherm shapes have changed little since 1974. The specific volume of Gasil-I has remained nearly constant during the ageing period but the pore size distribution (PSD) has shifted markedly to higher values. Electron micrographs show that low (room) temperature gas-solid ageing results in similar enlargement at the point of contact between attached secondary particles as that which occurs in hydrothermal ageing. In the gas-solid case, this change, which accounts for the decrease in overall surface area, is attributed to the surface transport of silica material in the presence of near monolayer quantities of adsorbed water. Ageing in this manner is geometry-limited so that the rate of ageing is expected to approach zero, resulting in time-stable silicas. (C) 2007 Elsevier B.V. All rights reserved.