This paper presents the first direct and simultaneous density and thickness measurements by X-ray reflectometry (XRR) of films that develop during aqueous alteration of the French SON 68 (R7T7-type) nuclear glass which is a multi-component silicate. The feasibility of XRR for this type of glasses is investigated by comparing the results with those obtained using other techniques. The influence of drying and the presence of crystalline phases on the XRR signal is studied. It is shown that XRR still yields useful data when a traditional method such as solution sampling cannot be used anymore. The evolution of the layers with alteration time was followed during 134 days. Three different steps are observed: an interdiffusion step, the formation of a homogeneous gel and finally an electronic density gradient (porosity and/or composition) within the gets at long reaction times. This density gradient could explain the protective properties of the gel. (C) 2003 Elsevier B.V. All rights reserved.