[en] We have measured and calculated the electron-positron momentum distribution of La_2-xSr_xCuO₄ samples for Sr concentrations of 0, 0.1,0.13, and 0.2 and Ba_0.6K_0.4BiO₃. The momentum distribution of all samples contained features derived from the overlap of the positron distribution with the valence electrons. In addition, discontinuities typical of a Fermi surface are seen in the superconducting samples. The form and position of these features are in general agreement with band theory for both La_2-xSr_xCuO₄ and Ba_0.6K_0.4BiO₃. However the evolution of the Fermi surface with doping in La_2-xSr_xCuO₄ differed significantly from expectations based on single electron band theories. (orig.)