 To improve biomass pretreatment sustainability, we explore processing in biphasic mixtures of water and CO2. Why CO2? Well, because it acts as a green, easily separable and catalytic co-solvent for water. It allows us to pretreat biomass at high solid contents, up to 40 weight percent, with no additional chemicals. Building on our previous work, we're now using a high pressure stirred one liter reactor that allows us to pretreat large biomass particles up to one centimeter in size and to explore the effect of mixing. While doing this, we noticed that when pretreating at a single temperature stage, we observed a systematic trade-off. When cellulose to glucose conversion yields increased, we saw a systematic decrease in hemicellulose conversion yields and at the same time an increase in unwanted degradation product, fur-for-all. And that's exactly a trend you don't want to see. To improve on this, we explored using the temperature-dependent hemicellulose depolymerization reaction sequence. During pretreatment, hemicellulose depolymerizes to soluble ligamers. Those soluble ligamers then depolymerize to single monomers, which only then will degrade to unwanted byproducts. This sequence implies that there's always going to be a lag between initial hemicellulose depolymerization and degradation product formation, because initially what's not there can't degrade. We take advantage of this lag by pretreating biomass at a high temperature for a short time and we then finish the job at a low temperature for a longer time. By using this method, we reverse the earlier discussed negative trend. We are able to simultaneously boost hemicellulose and cellulose yields while the formation of degradation products remains about the same.