In this study we have used high resolution parallel electron energy loss spectroscopy (PEELS) and X-ray excited optical luminescence (XEOL) to investigate the chemical nature of the luminescence centre in fresh and aged porous silicon. We find that regardless of the non-stoichiometric oxides which were observed by PEELS in our fresh porous silicon layers, Si-Si bonded material is involved in the luminescence process. However, in the case of aged porous silicon both Si-Si and Si-O bonded material are involved.