[en] Although the magnetic properties of the crystalline rare earth or yttrium compounds with 3d transition metals have been extensively studied, the ferromagnetic resonance (FMR) behaviour of these systems has been analysed in only a few papers. This letter presents the results of some FMR measurements on Y(Fesub(x)Cosub(1-x))₂ and Gd(Fesub(x)Cosub(1-x))₂ compounds. The determined spectroscopic splitting factors of transition metals in R(Fesub(x)Cosub(1-x))₂ compounds (R = Y or Gd) are given by the mean of the individual g values for pure cobalt and iron metals. The FMR behaviour of ferrimagnetic Gd(Fesub(x)Cosub(1-x))₂ compounds can be described using the Vangsness relation, which supposes essentially that the moments are localized. (Auth.)