During the plasma etching of SiO 2 surfaces, CF m , NF n , and their ions break Si-O bonds. The knowledge of the reactivity of these species allows the generation of the most efficient species, e.g. by laser-induced processes. Inefficient or surface-blocking species can be eliminated. The Si-O bond breaking by CF m (m = 4-1) and NF n (n = 3-1) is investigated by calculation of the cyclic transition structures. For these investigations, SiO 2 surface cut-outs with the attacked Si-O-Si bridges and Si-OH groups are modelled by finite molecular models. The structures of the reactants, products, and transition states are calculated using the Hartree-Fock method and the 3-21G( * ) basis set. Energies are estimated at the MP2/6-31G * * level. The calculated reaction and activation enthalpies provide the relative reactivities of the different species and surface structures. The results show that CF 2 is the most suitable molecule of the investigated ones for SiO 2 surface etching.