Experiments were carried out with a hexagonal array of 61 iron electrodes in sulfuric acid solution under conditions in which slow active-passive relaxation oscillations occur. The currents of the Individual electrodes were measured independently, and therefore the spatiotemporal patterns that occurred were directly determined. The potential of one or more of the electrodes was held at a value below that of the remaining electrodes : tbe electrodes at lower potential had a higher natural frequency and thus acted as pacemaker centers. Activation waves emanated from these pacemakers; the repassivation waves had the same characteristics as those on an array or single electrode held at a constant potential, e.g., from edge to center in the period-one parameter region. Long-range coupling plays an important role in the dynamics of electrochemical reactions; arrays of electrodes at constant potential behave qualitatively similar to single electrodes of the same total surface area. Both the size of the pacemaker centers (the number of electrodes held at a lower potential) and the difference in frequency between the pacemaker and nonpacemaker electrodes could be varied independently. Since conditions at positions across the reacting surface, the array, can be varied systematically, the array can be used to study the influence of pacemakers or model "defects" on a type of heterogeneous reacting system.