Thermal conversion of phosphorus (P)-rich waste materials such as sewage sludge offers several advan-tages: generation of bioenergy, concentration of plant nutrients and the destruction of organic pollutants.Different thermal processes modify the feedstock’s chemical and physical structure in differing ways,which also affects P speciation and plant availability in the residual ashes or carbonization products.This study assessed to which extent the P plant availability of ashes and chars produced from one batchof sewage sludge by incineration, pyrolysis or gasification was affected by particle size management andpost-process oxidation. Overall, a smaller particle size of the materials as well as post-process oxidationof non-oxidized materials increased the amount of plant-available P in the soil. In a pot experiment, allthe materials increased plant biomass compared with the untreated control, but the pyrolysis chars had asubstantially greater fertiliser value than the gasification ashes, while the two tested incineration ashesdiffered in their P fertilizing effect. P availability in non-oxidized materials was partly related to lowerprocess temperatures and lower levels of crystallinity. However, downstream oxidation simultaneouslyincreased crystallinity and P availability in a pyrolysis char and gasification ashes, resulting in an increasein plant P uptake of up to 60%. Results indicate that the oxidation of poorly soluble Fe-phosphates maycontribute to the positive effect on P availability. The results suggest that changes to the design and set-tings of the thermal conversion processes of sewage sludge offer considerable potential for improving Pavailability in the residual material.