The effect of elevated temperature and solvent exposure on the microstructure of auxetic polyurethane foams is investigated. It is shown that such effects result in an expansion of the auxetic foams accompanied by the removal of the highly convoluted features in the microstructure of auxetic foams with the result that such foams lose their auxetic characteristics. It is also shown that such changes do not occur if the foams are subjected to the high temperature or solvent exposure in a contained state which does not permit the expansion. This means that auxetic foams can still be used in such environments provided that the right precautions are taken (e.g. putting them in a cover thereby constraining them to retain their volume).