The oxidation of hydrogen-terminated porous silicon surfaces produced by electrochemical etching has been studied using transmission FTIR spectroscopy. The surface is passivated to oxidation by surface hydrogen below about 523 K. Above this temperature as hydrogen depletion occurs by H-2 evolution, Si surface dangling bond sites, capable of O-2 dissociation, are involved in initiating the first stage of oxidation. Two reactions are observed. The first, O insertion into Si-Si back-bonds, leads to -OySiHx surface species which exhibit frequency shifts to the blue compared to parent SiHx stretching modes. In addition, Si-O-Si modes are also observed to form. The second reaction involves oxygen atom insertion into Si-H bonds to produce isolated Si-OH surface species.