Abstract: A process to perform a controlled exothermic delignification of biomass comprises providing a system having a first vessel and a second vessel, providing a biomass comprising lignin, hemicellulose and cellulose fibers placed into the first vessel, providing an aqueous acidic composition comprising a sulfuric acid component, providing a modifier component and providing a peroxide component. The biomass is exposed to the sulfuric acid component and peroxide component, thereby creating a reaction mass. The reaction mass is mixed and the sulfuric acid, modifier and peroxide components are allowed to come into contact with the biomass for a period of time sufficient for a delignification reaction to occur and to remove a predetermined amount of the lignin from the biomass. The predetermined amount is assessed by testing a first kappa number of the biomass. When the first kappa number of the biomass is reached, the biomass is transferred to the second vessel.