The realization and preliminary testing of two types of ultramicroelectrode arrays consisting of 100 square-shaped Pt electrodes (5×5 μm, 25×25 μm) is described. In order to limit the number of external connections, the 'on chip' multiplexing approach is chosen. The matrix of 100 elements is addressed by 22 connections, using a basic NMOS analog multiplex. Test on the electronics show good performances for switching times, background current and signal-to-noise ratio. Electrochemical measurements in 1 M H2SO4 illusrate switching and multiplexing. Comparison of voltammograms of the Pt electrodes with those of bulk material reveal a good surface quality.