An important parameter controlling the mobility of melt layer in laser ablation of polymers is the viscosity. We report the first experiments to measure it at the extreme conditions of ultraviolet (UV) KrF laser ablation for poly(ethylene terephthalate) (PET). The volume of laser-induced bumps (re-solidified molten material expelled from the irradiated area to its periphery) is measured with AFM microscopy. The comparison of this value with an analytical modelling (based on the solution of Navier-Stokes equations for viscous melt flow induced by the gradient of ablation plume pressure with the use of experimentally measured melt depth) allows to calculate the kinematic viscosity of laser melt layer on the polymer surface. It appears to be about an order of magnitude greater than the kinematic viscosity of water at room temperature. (C) 2002 Elsevier Science B.V. All rights reserved.