Combustion of sewage sludge with subsequent recovery of phosphorous is a relatively new sludge treatment option. In the work described in this paper, recovery of phosphorus by acid leaching of fly-ashes from co-combustion of sewage sludge with wood in a circulating fluidized bed combustor was investigated. The results showed that it is possible to find a pH range (0.5-1) in which it is possible to extract a significant fraction of the phosphorous from these ashes without precipitation of secondary phosphates. The type of flocculation agent used in the waste water treatment plant where the sludge is formed has a significant effect on the phosphorous recovery. Ashes from combustion of sewage sludge that was formed using aluminium sulphate as flocculating agent released nearly all phosphorus at a pH-value of 1. When iron sulphate was used as flocculating agent, this affected the chemistry of the resulting ashes, making phosphorous recovery more difficult. The yield of phosphorous from those ashes was 50-80%. In part II of the project [1] focus is on the trace element release to the leachate. This determines if the leachate can be used directly as a fertiliser or if further removal of trace elements is necessary.Sludge, biofuel, phosphorus, leaching, co-combustion and fluidized bed 2