This paper describes the development of a downhole CO2 sensor that can continuously monitor changes in CO2 concentration under high pressure reservoir conditions. The sensor uses a Severinghaus-type design with a metal-oxide electrode, gas-permeable membrane, and internal electrolyte solution. Laboratory tests showed the sensor provided a linear response between output potential and dissolved CO2 levels under 1,000 psi pressure. Further CO2 injection coreflooding tests demonstrated the sensor could successfully monitor CO2 movement during underground storage processes.