The composition of oil seed rape straw treated by steam explosion with increasing severity was investigated before and after saccharification. Chemical changes were monitored by FTIR-ATR spectroscopy. Sugar contents were determined after acid hydrolysis. Discriminant analysis of the spectra before and after digestion showed the main compositional changes are losses of carbohydrates and a subsequent increase in the proportion of lignin. Construction of partial least squares (PLS) predictive models for the concentration of eight cell wall sugars indicated different fates for cellulose, hemicelluloses and pectic substances. No cellulose was lost during steam explosion and the amount digested to glucose increased linearly with severity. Pectin was partially degraded during steam explosion, but a bound fraction remained which was only released during saccharification. Hemicelluloses were gradually destroyed in the steam explosion process, and the extent of subsequent saccharification was most strongly associated with the breakdown of xylan-like hemicelluloses. (C) 2014 Elsevier Ltd. All rights reserved.