Early stages of cyclic fatigue-loaded polystyrene (PS) specimens were investigated by positron annihilation lifetime spectroscopy (PALS) at a maximum stress amplitude of 15 MPa. PALS yields information about the average unoccupied hole volume. A linear increase in the ortho-positronium (o-Ps) lifetime was observed in a range from 0 to 50,000 cycles. This increase occurs homogeneously distributed at different positions along a sample of 170 mm. The average unoccupied void volume increases by 1.2%. On the other hand, the o-Ps intensity shows no systematic change upon cycling. The results suggest a homogeneous and linear increase in free volume prior to craze formation. (C) 2008 Wiley Periodicals, Inc.