AbstractSome samples of technical grade silica and titania on storage for several years at room temperature show loss of apparent surface. The loss is attributed to dehydration of surface hydroxy‐groups. This can cause adhesion and aggregation of particles whereby the surface measured both by nitrogen and by dyes decreases, and, in some cases, causes a loss of surface reactivity.With heating at temperatures up to 250°, the loss of apparent surface of silica is increased, but that of titania is partly restored. It is suggested that these effects may influence specific surface measurements made by vapour phase adsorption if high temperature out‐gassing is empl