BACKGROUND: An efficient pretreatment for softwoods prior to enzyme hydrolysis is pressurised steaming of the chips with successive attrition by disc refining and ball milling following removal of some hemicelluloses in a compression screw. For the process to be economical, thermal and mechanical energy inputs need to minimised and sugar yield maximised, so the aim of this study was to improve understanding of the behaviour of fibres during each stage of the pretreatment. RESULTS: Radiata pine wood chips were treated with steam at 173 degrees C for between 3 and 144 min. The responses to refining and ball milling in terms of particle size reduction and sugar yield improved gradually as steaming time was increased to 36 min, but then an abrupt change in behaviour occurred resulting in more small particles during refining and much faster response to ball milling. This point coincides with an increase in cellulose degradation and fibre embrittlement. However, this higher steaming severity did not necessarily give highest sugar yields during saccharification as it limits the amount of energy that can be usefully applied during ball milling. CONCLUSIONS: The results from this study can be utilised for optimisation of the pretreatment of softwoods like radiata pine. While effective in delivering high sugar yields, the steaming/refining/ball milling pretreatment is quite energy intensive but the results of this study show that significant savings in overall energy are possible by optimising relative energy inputs to the steaming and ball milling stages. (C) 2016 Society of Chemical Industry