Spectra of positively charged secondary ions from thermally grown SiO2 films were recorded in a time-of-flight secondary ion mass spectrometry scheme. Ablation of cluster ions was induced by the impact of slow (4 keV/u) Au69+ projectiles. The intensities of SixOyHz+, (x=1-22, y=1-44, z=0-7) clusters are found to depend sensitively on the oxygen to silicon ratio and also on the hydrogen content. We find that oxygen rich clusters, y=2x+1, and, in one case, y=2x+2, can be stabilized by the incorporation of two additional hydrogen atoms in the cluster.