Sugar cane bagasse was pretreated with either liquid hot water (LHW) or steam using the same 251 reactor. Solids concentration ranged from 1% to 8% for LHW pretreatment and was greater than or equal to 50% for steam pretreatment. Reaction temperature and time ranged from 170 to 230 degreesC and 1 to 46 min, respectively. Key performance metrics included fiber reactivity, xylan recovery, and the extent to which pretreatment hydrolyzate inhibited glucose fermentation. In four cases, LHW pretreatment achieved greater than or equal to 80% conversion by simultaneous saccharification and fermentation (SSF), greater than or equal to 80% xylan recovery, and no hydrolyzate inhibition of glucose fermentation yield. Combined effectiveness was not as good for steam pretreatment due to low xylan recovery. SSF conversion increased and xylan recovery decreased as xylan dissolution increased for both modes. SSF conversion, xylan dissolution, hydrolyzate furfural concentration, and hydrolyzate inhibition increased, while xylan recovery and hydrolyzate pH decreased, as a function of increasing LHW pretreatment solids concentration (14%). These results are consistent with the notion that autohydrolysis. plays an important, if not exclusive, role in batch hydrothermal pretreatment. Achieving concurrently high (greater than 90%) SSF conversion and xylan recovery will likely require a modified reactor configuration (e.g. continuous percolation or base addition) that better preserves dissolved xylan.