The aim of this work was to understand the structural and microtextural changes responsible for microporosity formation in saccharose-based chars activated with CO2 at 850 degreesC. High-resolution transmission electron microscopy (HRTEM) and optical microscopy (reflectance measurements) allow to follow the changes in the chars organisation from nanometric to micrometric scales. Quantitative structural and microtextural data can now be extracted from the HRTEM images with an in-house image analysis procedure, Our results allow to suggest that the development of the microporosity with an increasing bum-off could be due to the development of slit-shaped pores. Such changes in pore shape and dimensions affect optical properties a relationship was established between BET surface area and mean reflectance, By coupling these experimental data and a theoretical approach, a model of activated carbons is proposed to explain the reflectance changes during activation. (C) 2002 Elsevier Science B.V. All rights reserved.