The development of carbon sensors to monitor and study ocean acidification is widely recognized as a research priority at this time in history. Dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO2) are desirable parameters for accurately characterizing the marine CO2 system. This project will develop a miniaturized, in-situ sensor, Channelized Optical System II (CHANOS II) for simultaneous, spectrophotometric measurements of seawater DIC and pCO2 with high frequency (~1 Hz) for up to 1000 m of water depth. Built on the success of the original CHANOS, CHANOS II will develop a miniaturized fluidic CO2 equilibrating manifold for both DIC and pCO2 channels, applying miniature pumps and switches to dramatically reduce sensor size, power consumption, and cost, and improve robustness. CHANOS II will be capable of making high-quality measurements on both stationary and mobile platforms, such as buoys, profilers, and autonomous underwater vehicles (AUVs). The deployable time depends on what platforms it will be on. For stationary platforms, it can be up to 4 months at hourly sampling interval; for mobile platforms, it can be up to one week continuous measurements.