The oxidation process of Co/AlOx/Co magnetic tunnel junctions has been investigated by photoconductance, in addition to traditional transport measurements. The shape of the photoconductance curves is explained within the framework of a simple qualitative model, assuming an oxidation time dependent imbalance of the incident forward and reverse hot electron fluxes, as well as inelastic scattering processes in the oxide. Due to the large sensitivity of the technique, the presence of unoxidized Al beneath the barrier layer can be monitored very accurately. The disappearance of a negative contribution to the photocurrent indicates the complete oxidation of the barrier layer, which coincides with the maximum magnetoresistance. From a Fowler analysis, the barrier height is determined as a function of oxidation time. The observed disagreement of the effective barrier heights determined by this technique and those found by Simmons fits demonstrates the added value of photoconductance studies.