Free-radical copolymerizations of ethylene (E) with methacrylic acid (MAA) and of E with acrylic acid (AA) were carried out in a continuously operated tank reactor at 2000 bar and temperatures up to 280 degrees C. The (M)AA content of the polymer, F-(M)AA, that may be reached by polymerization in homogeneous phase, e.g., at 260 degrees C, is below 10 mol-% and the associated (M)AA content of the monomer mixture is below 1 mol-%. Under such conditions, the ethene and the comonomer reactivity ratios, r(E) and r((M)AA), respectively, can not be simultaneously derived by fitting the measured F-(M)AA and f((M)AA) data according to the classical procedure. With r((M)AA), however, being available from a preceding C-13 NMR study(1)), r(E) may be calculated from each pair of experimental F-(M)AA and f((M)AA) values. F-(M)AA values are determined by means of elemental analysis of carbon and oxygen with consistency checks of copolymer composition brine performed via FT-IR spectroscopy on polymeric films. f((M)AA) is deduced from the measured monomer feed fluxes and from the amount and composition of the copolymer produced within given intervals of stationary polymerization. The r(E) data for the E/MAA and E/AA systems are presented and discussed. At identical polymerization pressure and temperature, the measured r(E) values of both systems agree within about +/-10%. The r(E) values, in addition, are very close to the ethene reactivity ratios that were recently measured for several E/(meth)acrylate copolymerizations. Arguments for this family-type behavior of r(E) are discussed.