Initiation of a localised plastic zone is numerically simulated using a constitutive model that incorporates an accurate description of the post-yield behaviour with the important phenomena of strain softening and strain hardening. Subsequent nucleation of voids in the deformation zone is detected using a hydrostatic stress criterion. This criterion is identified and validated. A micro-indenter with a sapphire sphere is used to produce indents that are later examined with an optical microscope. These observations lead to a critical force at which crazes are initiated in polystyrene. Combining these experiments with a numerical study shows that the loading part of indentation can be accurately predicted. A critical hydrostatic stress of 40 MPa is extracted from the numerical model by monitoring of the local stress field at the moment that the indentation force reaches the experimentally determined force level at which crazes were found to initiate. This criterion is validated by indentations on samples with different thermal histories, and at various loading rates. Finally, the influence of network density on the value of the hydrostatic stress criterion is investigated by indentation of blends of polystyrene and poly(2,6-dimethyl-1,4-phenyleneoxide). It is shown that the critical hydrostatic stress increases with network density. (C) 2003 Elsevier Science Ltd. All rights reserved.