The influence of the ionic liquid (IL) 1-ethyl-3-methylimidazoliumethylsulfate ([EMIM]EtSO4) on the polymerization kinetics of methyl methacrylate was investigated. ILs are liquids with relatively high polarities and viscosities. These two characteristic properties are strongly correlated with the rate coefficients of propagation k(p) and termination k(t) of polymerizations carried out in ILs. The rate constant of termination kt decreases when the concentration of ionic liquid, and thus the viscosity is increased, whereas the propagation rate coefficient k(p) increases with increasing IL content. The viscosity of ILs can be varied by either working with mixtures of ILs with conventional organic solvents - here the IL [EMIM]EtSO4 was mixed with dimethyl formamide (DMF) - or by variation of the temperature. The studies were carried out to determine the influence of the viscosity on the propagation and the termination reaction as well as the molecular weight distribution. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.