The free-radical copolymerization of ethene (E) and butyl methacrylate (BMA) is studied between 160 and 250 degrees C 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 degrees C and 2000 bar, are found to be 0,044 +/- 0,003 and 10,9 +/- 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 home-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 home-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.