An experimental method of determining the probability of fibre shortening in the form of a selection function during refining has been developed based on comminution models used in the crushing and grinding industries. The method was validated by comparing experimentally measured selection functions to theoretical estimates for a model refining process. Selection functions were experimentally determined for a series of pilot refining trials that examined five refiner plate patterns operating over a wide range of specific intensity and specific energy. We concluded that the probability of fibre cutting during refining is proportional to the applied energy and fibre length and that for a single refiner at constant specific energy the selection function increased with Specific Edge Load (SEL) and was approximately independent of feed consistency. For a constant specific energy, achieved using a wide range of flow rates, consistency, applied power and plate pattern, selection function was shown to strongly correlate with SEL. (C) 2003 Elsevier Science B.V. All rights reserved.