The present invention describes a method and device for pretreatment of organic material, more specific bio mass, for energy conversion, where said method comprises a first preheating step with a preheating vessel (4), a hydrolysis step with a hydrolysis reactor (5) and a pressure reducing step with a pressure reduction vessel (6), where the transfer of said organic material from the preheating vessel (4) to the hydrolysis rector (5) is effected by gravity and by creating a vacuum in the reactor (5). This method results in a very fast transfer of material from the preheating vessel (4) to the reactor (5). In addition, the filling volume of the reactor (5) is being controlled by a high frequency pressure sensor and supply of steam (3A) to the top of the reactor in order to provide the necessary head space. The invention also describes a device for performing said method.