The catalytic properties of (111) and (110) Pd50Cu50 single-crystal surfaces have been tested for the 1,3-butadiene hydrogenation reaction, and compared to those of pure Pd having the same surface orientations. The Pd50Cu50(111) single crystal has the highest activity; it is also more effective than the corresponding pure Pd face. The activity of the Pd50Cu50(110) is less; nevertheless, it is practically as effective as the Pd(110) crystal, which is itself more effective than the (111) oriented Pd sample. The results are discussed in terms of both geometric and electronic effects. They are associated on one side to the dilution by the inactive Cu surface atoms, and on the other side to the electronic interaction between the outer Pd surface atoms and the surrounding Cu atoms.