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The exchange stiffness constant A of evaporated (Gd,Tb)-(Fe,Co) amorphous alloys with constant Curie temperature of 465±25 K was determined with two different methods for a large composition and temperature range. One is the excitation of standing spin waves using Brillouin light scattering. The other method is the deviation of A from measurements of the wall energy density σw and the uniaxial anisotropy Ku using the Bloch wall energy equation. σw could be determined from the applied magnetic field difference for collapsing and expanding of thermomagnetically written domains. The compositional and experimental limits of this method are described. The uniaxial anisotropy Ku was measured with a torque magnetometer. In GdTb-FeCo the resulting exchange stiffness constant of A=(2.6±0.6)×10−12 J/m was found to be independent of the Tb content. The results obtained from Brillouin light-scattering measurements on Gd21.2 Co78.8 and Gd13.5Tb6.2Fe80.3 are in good agreement.