Using first principles calculations, we examine how the ensemble effect on the performance of bimetallic catalysts is affected by the change of surface electronic structure associated with their geometric parameters. We look at H2O2 formation from H2 and O2 based on three different Pd monomer systems including AuPd adlayers with a Pd monomer each on Pd(111) [AuPdM/Pd(111)] and Au(111) [AuPdM/Au(111) ] and a 55-Atom cluster with Au41Pd shell and Pd13 core [Au 41Pd@Pd13]. Our calculations show that H2O 2 selectivity tends to be significantly deteriorated in the Au 41Pd@Pd13 and AuPdM/Au(111) cases, as compared to the AuPdM/Pd(111) case. This is largely due to enhancement of the activity of corresponding surface Pd and its Au neighbors, while isolated Pd surface sites surrounded by less active Au are responsible for the H 2O2 formation by suppressing O-O cleavage. This study highlights that ensemble contributions in multimetallic nanocatalysts can be a strong function of their geometric conditions, particularly local strain and effective atomic coordination number at the surface, that are directly related to surface electronic states.