Selective partial hydrogenation of 1,3-butadiene (C4H6) to butene (C4H8) on hydrogen-precovered Pd(110) has been investigated using temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and high-resolution electron energy loss spectroscopy (HREELS). Below room temperature, C4H6 was molecularly chemisorbed on a hydrogen-precovered Pd(110) surface and was in the a-bonded chemisorption state. The hydrogenation Of C4H6 did not occur in the sparse domains of less than 0.25 ML. With increasing C4H6 coverage, C4H6 formed dense domains, and the a bond was weakened. Only the weakly a bonded C4H6 was hydrogenated to C4H8 at about 200 K. Because the stability of resultant C4H8 was low on the surface at the reaction temperature, C4H8 desorbed immediately after being produced or dehydrogenated to C4H6 if even a part of the C4H8 remained on the surface. We concluded that the balance between the hydrogenation reactivity Of C4H6 and the stability Of C4H8 leads to the partial hydrogenation on the Pd surface.