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Carbon paste electrodes modified with montmorillonite clay are used for the collection of iron( 111) from dilute solutions followed by voltammetric quantification o f the surface-bound cation. Open-circuit conditions are maintained during the ion exchange preconcentration. The surface is regenerated by a brief exposure t o a sodium carbonate solution. For 20 preconcentration/measurement/regeneration cycles, the response could be reproduced with 4.8% relative standard deviation. Factors influencing the response, such as electrode composition, preconcentration time, measurement and "cleaning" solutions, ionic strength, coexisting metals, iron concentration, and other variables, have been investigated. A detection limit o f 0.2 mg/liter was estimated following 3 min preconcentration. The unusual mechanisms o f ion exchange processes at clays should lead t o a new CkdSS o f modified electrodes for preconcentration/voltammetric measurements.