The highest overall yield of hemicellulose and cellulose sugars obtained so far with one-stage pretreatment of a mixture of 70% fir and 30% pine is 75% , which was reached after impregnation with 0.65% H2SO4 followed by pretreatment at 215°C for 100 s. For spruce, the highest overall yields are 66 and 67% of the theoretical value using impregnation with SO2 and H2SO4, respectively . The most significant difference between pretreatment using H2SO4 and SO2 as a catalyst is the fermentability . Material that has been impregnated with H2SO4 is often poorly fermented after being treated at high severities while this is