An anomalous magnetic phase diagram for dilutions of UB4 with YB4 had been previously established. The (Y1-xUx)B4 system is paramagnetic for x 0.6, ferromagnetic for 0.1 < x < 0.6 and paramagnetic for x < 0.1. Measurements of the resistivity and magnetic susceptibility for 1 K < T < 300 K and the low temperature specific heat are presented. The observed behavior is suggestive of a localization of the 5f electrons upon dilution of UB4 by YB4 due to the reduction of the 5f-5f overlap. © 1985.