A new imaging method is presented enabling to monitor the lateral wet thermal oxidation of a thin Alcontaining layer embedded in a VCSEL structure. This method is based on the measurement of the modification of the VCSEL reflectivity spectrum inherent to the aperture layer refractive index change, with an observation window restricted to the wavelength ranges for which this reflectivity variation is maximal. The main purpose is the accurate control of the buried confinement aperture, and thereby that of the electro-optical characteristics of the laser device. The kinetics of the lateral oxidation has been studied for small size aperture VCSEL (3-10 µm) and for long range oxidation depths. This straightforward method based on an optical imaging system will enable to robustly improve production yield of this multifactor dependent technological process.