Dilute sulfuric acid pretreatment of corn stover has been demonstrated in the National Renewable Energy Laboratory’s bench scale batch pretreatment reactor and 1/4 tonne/day continuous horizontal pretreatment reactor.  Operation of the continuous reactor has resulted in lower xylose yield and higher degradation product formation compared with the batch reactor at same conditions.  Independent testing of the continuous reactor has revealed a broad residence time distribution, which explains reduced pretreatment performance as unconverted xylan and xylose degradation products result from insufficient and excessive reaction time, respectively.  To improve residence time control by reducing back mixing and removing stagnant areas, the reactor’s original interrupted-flight auger and smooth tube shell have been replaced with a continuous-flight auger and tube containing anti-rotation bars.  The measured residence time distribution and corresponding pretreatment performance of both continuous reactor designs will be discussed and recommendations will be provided.