The electron momentum distribution in icosahedral Cd 84 Yb 16 has been studied using the high-resolution Compton scattering technique with a momentum resolution of 0.16 a.u. The experimental valence-electron Compton profile is decomposed into two components: an inverted parabolalike one and a broad Gaussian-like one. We have found that the Fermi sphere, deduced from the number of electrons under the inverted parabolalike component, just coincides with the quasi-Brillouin zones constructed from the intense ͑211111͒ and ͑221001͒ reciprocal points. The Gaussian-like part is attributed to the electron occupation of the Yb 5d states. These facts are taken as signature that both the Hume-Rothery mechanism and the sp-d hybridization mechanism contribute to the formation of the pseudogap, stabilizing the icosahedral phase of Cd 84 Yb 16 .