Block copolymer (BCP) micelles formed by polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) were used as nanoreactors for preparing stable bimetallic AuAg nanoparticles (NPs) with finely tunable composition and diameters. Micelles loaded with NPs were then deposited onto TiO 2 and Al 2 O 3 in order to investigate the catalytic properties of the NPs with respect to CO oxidation. The polymer matrix could be removed by thermal treatment at 400°C under H 2 or O 2 . Different parameters, such as support, particle size, and metal composition were varied independently, so that their influence on the catalytic activity for CO oxidation could be separated. The atomic Au/Ag ratio was varied from 1:2 to 2:1, and the highest activity was obtained for an Au/Ag ratio of 1, showing synergy effects of both metals for catalyzing CO oxidation. Using nanosized TiO 2 as support and an Au/Ag ratio of 1, the nanoparticle diameter was varied between ∼3 and ∼20nm which led to a variation of activity by a factor of ∼7 (in a continuous flow reactor at 70°C) with the smallest particles showing the highest turnover frequencies (TOFs). In comparison, regular TiO 2 as catalyst carrier showed significantly lower performance than nanostructured TiO 2 while no activity was found on non-reducible γ-Al 2 O 3 . Independently, all three parameters (metal particle size, Au/Ag atomic ratio, and the support) showed significant effects on the catalytic activity.