The Mn < 0.4 panel of Fig. is most central to the goal of achieving high activity at a composition enriched with elements that typically resist corrosion. Considering only Mn concentrations below 0.4, the highest activity (current density at 1.79 V vs RHE) from the Mn-Sb-O baseline system is 1.1 mA cm -2 from the Mn0.38Sb0.62 sample. The highest activity from addition of Sn, Ti, and Sn+Ti are as follows: 2.4 mA cm -2 from the Mn0.34Sb0.44Sn0.22 sample, 35 mA cm -2 from the Mn0.39Sb0.24Ti0.37 sample, and 37 mA cm -2 from the Mn0.40Sb0.22Sn0.08Ti0.30 sample, respectively. These correspond to activity improvements at 1.79 V vs RHE by factors of approximately 2, 32, and 34, respectively. The cyclic voltammograms of these exemplar compositions are shown in Fig. . The quinary oxide space contains 9 composition samples with Mn concentration below 0.3 and current density above 1 mA cm -2 . These compositions include Sb concentrations between 0.40 and 0.48, Sn concentrations between 0.07 and 0.23, and Ti concentrations between 0.08 and 0.24. This Mn concentration is below that of the quintessential MnSb2O6 rutile structure that has been central to acid OER studies in this family of catalysts. While the combinatorial exploration of Mn-Sb-Sn-O, Mn-Sb-Ti-O, and Mn-Sb-Sn-Ti-O composition spaces illustrates that both Sn and Ti can help achieve high activity at low Mn concentration, the data also suggest that the global maximum in these compositions spaces has yet to be identified. The design of the experiments based on improvements upon the Mn-Sb-O baseline resulted in biasing exploration of each higherorder composition space to compositions near this baseline, whereas the highest activities are observed with substantial concentrations of Sn and/or Ti, motivating future exploration of compositions with even higher concentrations of these elements.