The effect of process conditions used for wheat straw pretreatments on the liquor- and residue-composition was studied. Hereto, the pretreatment conditions were expressed in a 'combined severity R'(0)-factor'. The higher the combined severity factor (R'(0)) the more xylan was released from the wheat straw, but the more xylan decomposed and furfural formation occurred. The percentage of residual xylan present after pretreatment appeared to be a good indicator concerning cellulose degradability or bio-ethanol production. Namely, cellulose degradation by using commercial enzymes was higher at higher severities corresponding to a lower amount of residual xylan. The xylan release and degradation was studied in more detail by using HPSEC and MALDI-TOF mass spectrometry. The more severe the treatment the more (acetylated) xylose oligomers with a DP lower than nine were analysed. The presence of (acetylated) xylans with a DP of 9-25 increased slightly from low to medium severity. The quantification of the DP-distribution of the (acetylated) xylans released proved to be a good tool to predict cellulose degradability. (c) 2006 Elsevier Ltd. All rights reserved.