In this work we have used low temperature photoluminescence (PL) to study a set of CdS/CdTe solar cells grown by close space sublimation. The samples were given a post-growth treatment at 390 degreesC with HCl in a varying partial pressure of oxygen of up to 500 mbar. By using a focussed optical probe beam, spots on the cells similar to300 mum in width were identified, that had brighter PL than 'normal' regions. Since PL bands from such regions were shifted to lower energy, they were identified as having enhanced diffusion between CdS and CdTe. Using PL assignments from the literature, the evolution of CdTe and CdS PL bands with increasing oxygenation was interpreted in terms of the active centres present. It was postulated that oxygenation: (i) enhances concentrations of Cl-Te and V-Cd in CdTe, (ii) enhances diffusion of Te into CdS, with highly oxidising conditions yielding Te-O complexes, and (iii) can act to fill V-S with O-S. These observations provide a starting point for understanding the role of oxygen in oxygen/chloride co-processing. (C) 2003 Elsevier Science B.V. All rights reserved.