Mesoporous layers of silica are of interest for many optical applications, including the creation of devices ranging from optical sensors to laser sources with promising characteristics. Their optical characterization goes through the interpretation of measurements. We propose to give experimental results of reflectance and scattering measurements concerning mesoporous layers of silica deposited onto silicium substrates. This work shows that their interpretation in terms of optical indices or dielectric function have to be taken with care since scattering is present, and thereby the Kramers-Kronig relations appear not to be fulfilled. (c) 2005 Elsevier B.V. All rights reserved.