Dilute acid pretreatment of cellulosic biomass can give high yields of dissolved xylose monomers and oligomers and highly digestible cellulose in the residual solids.  A much greater fraction of oligomers results at higher pH, and overall monomer plus oligomer yields drop.  Oligomer solubility and reaction selectivity could help explain differences in hemicellulose hydrolysis yields, but little data is available with which to estimate their importance.  Our measurements of the solubilities of highly pure xylooligomers of chain lengths (6-12) produced from high temperature, water-only hydrolysis of birchwood xylan showed that solubility decreased with increasing chain length, with values at 31.1°C dropping from 30.3 to 3.7% over a chain length range from 7 to 12.  In addition, higher degree of polymerization (DP) oligomers precipitated out of solution before lower DP species when a mixed solution of hemicellulose hydrolysis oligomers was cooled.  Our measurements of xylose, xylobiose, xylotriose, xylotetrose, and xylopentose disappearance at 160^oC over a pH range from near neutral to 1.45 showed that the selectivity of xylose formation from xylooligomers increased appreciably with acid concentration and decreased with increasing DP at a given acid concentration.  Overall, our findings suggest that oligomer solubility is not a critical factor at reaction temperatures for hemicellulose hydrolysis but that higher DP xylooligomers that are soluble at reaction temperatures precipitate onto solid biomass when cooled, limiting yields of dissolved product.  Furthermore, the change in selectivity with pH is consistent with an optimum acid concentration that maximizes xylose recovery for acid hydrolysis of xylan rich hemicellulose.