Free-radical copolymerization of ethene (E) and methyl methacrylate (MMA) has been studied between 190 and 290°C at 2 000 bar. The reactions were carried out in a continuously operated device at overall monomer conversions mostly below 1.5%. Monomer feed concentration is obtained from the measured mass fluxes. Copolymer composition is determined via elemental analysis. Reactivity ratios, rE and rMMA, are derived from non-linear least squares fitting to the Mayo-Lewis equation of the compositions of monomer mixture and copolymer. E. g., at 220°C and 2 000 bar, rE and rMMA are found to be 0.056 ± 0.004 and 10.0 ± 1.1, respectively. Copolymerization at around 250°C and above to products containing more than 40 mol-% MMA units is affected by depropagation. This is shown by fitting monomer mixture vs. copolymer composition data up to different MMA contents. The activation energies of both reactivity ratios, EA(rE) and EA(rMMA), are compared with the associated differences in activation energies reported by the Hanns Fischer group for the addition reactions in solution of appropriate small radicals to E and to MMA. The data from the two sources are found to be in excellent agreement which suggests that the temperature dependence of a wide variety of reactivity ratio data might be accessible from the extended set of activation energies of radical-molecule addition reactions studied by Fischer and coworkers. At 220°C, rE is also measured for 1 500 and 2 500 bar. As has already been seen with the E-BMA system, the arithmetic mean value of the associated homo-propagation activation volumes allows for an estimate of the pressure dependence of cross-propagation rate coefficients which in turn provides information about the variation of reactivity ratios with pressure.