Pretreatment is an essential, but expensive, step in biologically converting biomass to fuels and chemicals, and acids added or released during the reaction hydrolyze hemicellulose to sugars and expose cellulose for enzymatic digestion with good yields. Pretreatment by liquid flow past solids increases hemicellulose removal yields particularly at higher flow rates, but current first-order homogeneous kinetic models indicate that the amount of water should not be important. However, these models suffer from inconsistencies, and the effect of varying sugarcane bagasse concentrations on xylose monomer and oligomer yields was experimentally measured in a batch reactor without adding acids or other chemicals at 200 degreesC. A greater drop in pH was observed at higher solids concentrations, as anticipated. Furthermore, only about 7-13% of the total xylose recovered in solution was as monomers at the maximum total xylose yield point, with the rest being oligomers, and although monomer yields could be increased at longer hold times, overall yields declined. These results and the general yield versus time profiles are consistent with the predictions of first-order models. However, a possible trend toward greater yields was observed at lower solids concentrations, but a paired difference test showed that these yield differences were only statistically significant between the extremes in biomass concentrations.