Mathematical models and experimental data are discussed for high-pressure ethylene polymerizations in stirred autoclaves and tubular reactors. The models are based on free radical mechanism of ethylene polymerization using material and energy balance equations as well as impact balance (for tubular reactors). A stirred autoclave reactor is modeled taking into account mixing modes in the reactor on the basis of the segregation approach. The developed mathematical model was used to study the stability of stirred autoclave reactor and determine the range of the process parameters corresponding to their sustainable functioning. (c) 2007 Elsevier B.V. All rights reserved.