This paper presents a readout circuit for a carbon dioxide (CO 2 ) sensor that measures the CO 2 -dependent thermal time constant of a hot-wire transducer. The readout circuit periodically heats up the transducer and uses a phase-domain <inline-formula> <tex-math notation="LaTeX">\Delta \Sigma </tex-math></inline-formula> modulator to digitize the phase shift of the resulting temperature transients. A single resistive transducer is used both as a heater and as a temperature sensor, thus greatly simplifying its fabrication. To extract the transducer's resistance, and hence its temperature, in the presence of large heating currents, a pair of transducers is configured as a differentially driven bridge. The transducers and the readout circuit have been implemented in a standard 0.16-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> CMOS technology, with an active area of 0.3 and 3.14 mm 2 , respectively. The sensor consumes 6.8 mW from a 1.8-V supply, of which 6.3 mW is dissipated in the transducers. A resolution of 94-ppm CO 2 is achieved in a 1.8-s measurement time, which corresponds to an energy consumption of 12 mJ per measurement, >10<inline-formula> <tex-math notation="LaTeX">\times </tex-math></inline-formula> less than prior CO 2 sensors in CMOS technology.