Combustion of sewage sludge with subsequent recovery of phosphorus 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 one can extract a significant fraction of the phosphorus 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 phosphorus 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 phosphorus recovery more difficult. The yield of phosphorus from those ashes was 50-80%. In Part II of the project [Pettersson et al. Leaching of ashes from co-combustion of sewage sludge and wood. Part II: The mobility of metals during phosphorus extraction. Biomass and Bioenergy 2007 in press, doi:10.1016/j.biombioe.2007.09.006 [1].] focus is on the trace element release to the leachate. This determines if the leachate can be used directly as a fertilizer or if further removal of trace elements is necessary. (c) 2007 Elsevier Ltd. All rights reserved.