High-throughput bioprocessing is a very promising technique for bioprocess development and optimization because of its high efficiency. The key to its development has been the availability of simple and inexpensive sensors to monitor the bioprocesses conducted in its small-scale bioreactors. Here we report on a low-cost noninvasive CO2 sensing system suitable for any transparent vessel. The system was composed of a CO2 sensing patch, a coaster, an interface, and a computer. The sensing film was prepared using the ion-pair technique. The coaster was a small self-made device with necessary optics and electronics for ratiometric measurements with a component cost of less than $100. Results show that the system was stable and reliable despite its simplicity and low cost. The sensitivity of the CO2 sensing system was not affected by pH, media type, or temperature. It was shown to be stable for at least 10 days, long enough for most bioprocesses. (C) 2004 Wiley Periodicals, Inc.