The free-radical copolymerization of ethene (E) and butyl methacrylate (BMA) is studied between 160 and 250 8C at 2000 bar. The reactions which were induced thermally are carried out in a continuously operated device at total monomer conversions mostly below 1%. Monomer feed concentrations are obtained from measured mass fluxes. Copolymer composition is determined via elemental analysis. Reactivity ratio data, r E and r BMA , are derived from non-linear least squares fitting of the monomer mixture and copolymer compositions. r E and r BMA , e. g. at 190 8C and 2 000 bar, are found to be 0,044 l 0,003 and 10,9 l 1,1, respectively. Simulation studies suggest that depropagation of BMA units has no major influence on the copolymerization kinetics at the reaction conditions of the present study. The r BMA data are clearly different from r A , the acrylate reactivity ratio for E-methyl acrylate, E-butyl acrylate, and E-2-ethylhexyl acrylate copolymerizations. By adopting the simplifying terminal model, from reactivity ratios and from extrapolated homo-propagation rate coefficients, cross-propagation rate coefficients are derived. The activation energy of such coefficients primarily reflects the type of monomer molecule whereas the free-radical chain-end influences the pre-exponential factor. A few experiments were carried out to estimate the pressure dependence of r E . It turns out that the arithmetic mean of homo-propagation activation volumes provides a reasonable estimate of the pressure dependence of cross-propagation. The available data for r E and r BMA allow to model monomer concentration vs. polymer composition behavior of free-radical E-BMA copolymerization at technically relevant temperatures and pressures.