According to an example aspect of the present invention, there is provided a method for prehydrolysing lignocellulosic chip material in a reaction vessel by a prehydrolysis reaction carried out in the presence of steam at a reaction temperature. The method comprises the steps of heating the contact surfaces of the reaction vessel above the boiling point of water by feeding steam flow into the reaction vessel, introducing during a preselected period of time a liquid SO2 flow into steam flow and vaporizing the liquid SO2 flow to form gaseous SO2 in the steam flow, continuing the feed of the medium pressure flow into the reaction vessel after said preselected period of time, and maintaining the reaction temperature for hydrolysing the lignocellulosic material.