As the pretreatment process is the most expensive and energy-consuming step in the overall second generation bioethanol production process, it is vital that it is studied and optimized in order to be able to develop the most efficient production process. The aim of this paper was to investigate chemical and physical changes in biomass during the process of applying the explosive decompression pretreatment method using two different gases—N₂ and synthetic flue gas. The explosive decompression method is economically and environmentally attractive since no chemicals are used—rather it is pressure that is applied—and water is used to break down the biomass structure. Both pre-treatment methods were used at different temperatures. To be able to compare the effects of the pretreatment, samples from different process steps were gathered together and analysed. The results were used to assess the efficiency of the pretreatment, the chemical and physical changes in the biomass and, finally, the mass balances were compiled for the process during the different process steps of bioethanol production. The results showed that both pre-treatment methods are effective in hemicellulose dissolution, while the cellulose content decreases to a smaller degree. The high glucose and ethanol yields were gained with both explosive pretreatment methods at 175 °C (15.2⁻16.0 g glucose and 5.6⁻9.0 g ethanol per 100 g of dry biomass, respectively).