We are developing a cell-based biosensor consisting of a planar microelectrode array that allows detection of extracellular potentials and their modulation in the presence of toxins or other active agents. To improve cell-electrode coupling, the microelectrodes were electroplated with platinum black. We report on the use of imaging x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), impedance measurements, and extracellular recordings to assess the effectiveness of this procedure. SEM provided highly detailed images of the shape and structure of well-formed deposits of thickness on the order of 1 mu m or more. Because of its inherent high surface sensitivity, imaging XPS could reveal the presence of platinum deposits that were too thin to be detected by SEM. For typical, well-plated microelectrodes, impedance measurements showed reductions in the electrical resistance at 100 Hz from roughly 60 M Ohm or more 1 M Ohm. The overall electronic coupling of biopotentials to the microelectrodes was demonstrated by recordings obtained from beating rat myocytes and from rat spinal cord cells.