Expanded polystyrene (EPS) beads have a cellular microstructure with closed cell membranes made of polystyrene. After demoulding, the intracellular temperature and pressure decrease down to room temperature and to atmospheric pressure. It is here shown that after processing, EPS shrinkage and after-shrinkage can be partly correlated to the intracellular pressure decrease, assuming that polystyrene is viscoelastic. To do so, use is made of micromechanical techniques in non-isothermal, linear, non-ageing viscoelasticity.