Using H-- and He+-ions in the energy range between 100 keV and 10 MeV. the effects of irradiation on polymethylmethacrylate (PMMA) were extensively studied. The surface shrinking phenomenon due to outgassing and the diffusion of the volatile scission products caused by the irradiation process have been investigated as a function of ion energy, ion fluence, dose rate and thermal treatment after the irradiation. Diffusion constants. mean diffusion path lengths and activation energies have been deduced from the shrinking dependency on the temperature. The influence of the ion fluence. the dose rate distinguished in current density and scan density and the particle energy is discussed. With respect to applications in deep ion beam lithography (DIBL) the results lead to the conclusion that one should prefer a rather dispersed instead of a sharply collimated beam in order to prevent an abrupt rupture during the irradiation. On the other hand. long irradiation times should be avoided to prevent inaccuracies with respect to the depth of the microstructures due to shrinking of the surface during irradiation. (C) 2002 Elsevier Science B.V. All rights reserved.