A potential advantage of applying two different acid concentrations in a percolation reactor was investigated both theoretically and experimentally. In the proposed mode of operation, a lower acid concentration was applied first to recover the easily hydrolyzable hemicellulose, then a higher acid concentration to recover the remaining hemicellulose. The reaction-conditions applied were 160-180 degrees C and 0.05-0.2% of sulfuric acid. The model predicted that the step-change operation of acid concentration increased the product yield by 3-4% over that of uniform acid concentration. The difference in yield was not fully verifiable experimentally since it was barely over the experimental error range. However, a significant difference was found in sugar decomposition, the step-change process showing much less furfural formation.