A voltammetric sensor developed for in situ trace metal analysis in natural waters is presented. It consists of an array of 100 mercury-plated, iridium-based microdisk electrodes, coated with a 300−600-μm-thick 1.5% agarose gel membrane. This membrane acts as a dialysis membrane by allowing the diffusion of metal ions and complexes and by hindering the diffusion of colloids and macromolecules. Chronoamperometry and square wave anodic stripping voltammetry (SWASV) have been used to characterize the diffusion of hexacyanoferrate(III), lead, and cadmium in the agarose gel. For these species, the diffusion coefficients have been found to be half of the diffusion coefficient in free solution, and the time necessary for complete equilibration with the test solution varied with the gel thickness in accordance with the theory and can be lowered to 5 min for a gel thickness of 300 μm. The same techniques have been used to demonstrate the efficiency of the membrane against fouling and convection. Pressures in the r...