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Thermal diffusion and oxidation processes are used in semiconductor and photovoltaic device manufacturing since decades and are to a large extent understood. However, a closer look reveals that not all aspects are well explained in literature. We observe for POCl3 diffused surfaces (drive-in at 860 °C) on wafers with different surface morphology (textured vs. non-textured) and crystal orientation ( vs. ) after subsequent dry thermal oxidation systematically a retrograde phosphorus P concentration profile in the first surface-near 10-20 nm. The retrograde part of the profiles is in contradiction to the pile-up of phosphorus on the Si side of the Si/SiO2 interface that is described in literature [1, 2] to be the consequence of the respective segregation coefficients. The observation is not a measurement artefact but the result of the thermal oxidation process. ECV and SIMS measurements have been cross checked with four-point probe sheet resistance measurements. In addition, SENTAURUS simulations have been performed to back up the observation. We conclude that the observations should result in new models for thermal processing (diffusion and oxidation) that might impact future solar cell process optimization.