[en] Fermi surfaces (FS) of binary Cu alloys (Cu-Ga, Cu-Ge and Cu-Ni) are investigated by use of the method of positron annihilation. Especially the dependence of the neck radii of the FS on the alloy concentration is examined. The rotating-specimen-method is used with a constant deviation angle (theta = 0₀) combined with a crossed-slit-geometry which yields a resolution of 0.9 x 1.7 mrad₂. To improve the intensity, up to 72 collimators are used simultaneously in an special sandwich arrangement. In the case of Cu-Ga and Cu-Ge the radius of the <111> neck Ksub(N) is an unique, monotonous function of the electron per atom ratio, e/a. This is a variance with the present measurements. In the case of Cu-Ge Ksub(N) increases upon alloying, i.e. with increasing e/a. In this regard qualitative agreement is obtained with Jones' model. But for the given increase of e/a, Ga widens the neck of the FS about 30% more than Ge does, though the maximum solubilities are at nearly identical e/a ratios. In the case of Cu-Ni the present measurements show that the neck radius of the FS does not decrease with increasing Ni concentration, in the contrary Ksub(N) (Cu-22at% Ni) is about 18% larger than Ksub(N) (Cu). This is attributed to the fact, that Ni modifies the density of electron states. The effect of the Ni 3d stated have to be taken into consideration. (orig.)