Oxygen evolution electrocatalysts in acidic media were studied by scanning electrochemical microscopy (SECM) in the substrate generation-tip collection (SG-TC) imaging mode with a 100 microm diam tip. Pure IrO2 and Sn(1-x)Ir(x)O2 combinatorial mixtures were prepared by a sol-gel route to form arrays of electrocatalyst spots. The experimental setup has been developed to optimize screening of electrocatalyst libraries under conditions where the entire array is capable of the oxygen evolution reaction (OER). The activity of individual spots was determined by reducing the interference from the reaction products of neighboring spots diffusing to the tip over the spot of interest. A gold layer deposited on the external wall of the SECM tip was used as a tip shield. In this study the shield was kept at a constant potential to reduce oxygen under mass transfer controlled conditions. The tip shield consumes oxygen coming from the neighbor spots in the array and enables the tip to correctly detect the activity of the spot below the tip. Simulations and experimental results are shown, demonstrating the effectiveness of the tip shield with the SG-TC setup in determining the properties of the composite materials and imaging arrays.