This paper describes the design, modeling, and experimental characterization of an electrochemical sensor array for on-line monitoring of fermentor conditions in both miniaturized cell assays and in industrial scale fertnentations. The viable biomass concentration is determined from impedance spectroscopy. As a miniaturized electrode configuration with high cell constant is applied, the spectral conductivity variation is monitored instead of the permittivity variation. The dissolved oxygen concentration is monitored amperometrically using an ultra-microelectrode array, which is shown to have negligible flow dependence. pH is monitored using an ion-sensitive field effect transistor (ISFET), and a platinum thermistor is included for temperature measurements. All sensors were shown to be sufficiently accurate within the range relevant to yeast fermentations. The sensor array is shown to be very stable and durable and withstands steam-sterilization.