High-field magnetization studies performed at 4.2 K in magnetic fields up to 35 T on amorphous Gd x Co 80− x B 20 and Er x Co 80− x B 20 alloys have revealed, for samples with stoichiometry close to that of a compensated ferrimagnet, a magnetic behaviour that is characteristic of a non-collinear magnetic structure of the rare-earth and cobalt sublattices. From the non-collinear regime the exchange interactions between the 3d and rare-earth magnetic sublattices have been accurately evaluated. The compositional dependence of the Co and rare-earth moments is discussed.