A combined experimental and theoretical study shows that the photooxidative activity of two isostructural metal oxide clusters depends on their internal templates. To this end, two halide-templated bismuth vanadium oxide clusters [X(Bi(dmso)3)2V12O33]− (X=Cl−, Br−) are reported and fully characterized. The two clusters show similar absorption features and illustrate that bismuth incorporation results in increased visible-light absorption. Significantly higher photooxidative activity is observed for the bromide-templated cluster compared with the chloride-templated one. Detailed photophysical assays and complementary DFT calculations suggest that the more efficient triplet excited state formation in the Br−-containing cluster is the decisive step in the photocatalysis and is due to the heavy-atom effect of the bromide. This concept can therefore open new pathways towards the optimization of photocatalytic activity in metal oxide clusters.