A process for pretreatment of a biomass material, comprising subjecting a biomass material to a biphasic mixture of water and supercritical CO2 at a temperature in the range of 150° C. to 250° C. under high pressure for a time of from 10 seconds to 100 minutes. In particular embodiments, the process is performed as a two-stage temperature process wherein an initial short high-temperature stage is conducted at a temperature of at least 200° C. for up to 20 minutes and a subsequent longer low-temperature stage is conducted at a temperature of at least 140° C. and up to 190° C. for 10-120 minutes.