NOVELTY - Reactor system comprising first reaction zone having first set of reaction conditions adapted to drive formation of reversible acid from one or more components in vapor phase of the reactor system, second reaction zone having second set of reaction conditions adapted to drive separation of the reversible acid to its components, biomass input adapted to introduce a biomass into the first reaction zone, and biomass output adapted to discharge the biomass from the second reaction zone.    USE - The reactor system and method are useful for treating biomass (all claimed).    ADVANTAGE - The reactor system eliminates or minimizes subsequent processing steps of components (steps including removal from the pretreatment reactor, recovery and recycling).    DETAILED DESCRIPTION - Reactor system comprising first reaction zone having first set of reaction conditions adapted to drive formation of reversible acid from one or more components in vapor phase of the reactor system, second reaction zone having second set of reaction conditions adapted to drive separation of the reversible acid to its components, biomass input adapted to introduce a biomass into the first reaction zone, and biomass output adapted to discharge the biomass from the second reaction zone, where the reactor system is adapted to move the biomass from the first reaction zone to the second reaction zone, and the reactor system is adapted to move one or more components of the reversible acid from the second reaction zone to the first reaction zone for formation of at least a portion of the reversible acid. An INDEPENDENT CLAIM is included for method for treating biomass, involves (a) (i) providing biomass containing polysaccharides to a first reaction zone having a first set of reaction conditions suitable to drive formation of a reversible acid from one or more components, (ii) treating the biomass by allowing the biomass to contact the reversible acid, (iii) moving the biomass in contact with the reversible acid from the first reaction zone to a second reaction zone having a second set of reaction conditions suitable to drive separation of the reversible acid to its components, and (iv) moving one or more components from the second reaction zone to the first reaction zone for formation of at least a portion of the reversible acid, or (b) (i) providing biomass containing polysaccharides to first reaction zone and forming reversible acid from one or more components in the first reaction zone, (ii) contacting the biomass with the reversible acid in the first reaction zone, (iii) moving the biomass in contact with the reversible acid from the first reaction zone to second reaction zone and separating the reversible acid into one or more components in the second reaction zone, and (iv) moving one or more components from the second reaction zone to the first reaction zone for formation of at least a portion of the reversible acid; and #product obtainable by the method.