A pretreatment process to remove potassium from straw fuel may be based on pyrolysis followed by char wash. The straw is pyrolysed at moderate temperatures at which the potassium is retained in the char. Potassium and residual chlorine are extracted from the residual char by water. Char and pyrolysis gases may then be used in a conventional boiler without problems due to the high straw potassium content. To evaluate this pretreatment process knowledge about the char wash process is needed. In this study wheat straw chars were experimentally investigated in the laboratory to determine the extraction time of potassium and chlorine during char wash with water. The influence of particle size, water temperature, straw type, potassium content in water and pyrolysis conditions were investigated. Based on the experimental data a mathematical model describing the extraction, was developed. The laboratory experiments showed that three fractions of potassium in the straw reacted differently: 35-58% of the char potassium was dissolved very fast, followed by a secondary slow potassium release that was strongly influenced by particle sizes, water temperature, char type and water KCl content. The residual 5-10% of the char potassium remains in the char and could not be removed with pure water.