Ab initio calculations have been performed to study the stability of isocyanate complex (NCO) over silica and alumina surfaces. Mulliken and natural bond population analysis methods have been used in order to analyze charge distributions and the direction of charge transfer processes. The results indicate that the NCO group adsorbs linearly and perpendicularly to both surfaces; namely, on top site over silica and on bridge site over alumina. It was observed that the charge transfer from the oxygen lone pairs of NCO to the N-C antibonds produces a weakening of N-C link. This phenomenon is more important over alumina, yielding to an easier NCO decomposition over the surface of this oxide.