Two related continuous-flow sensing strategies are presented for the determination of CO 2 (g) in gas mixtures. A continuous monitoring scheme is described for the determination of the analyte in atmospheric air, and a discrete sample introduction approach for the determination of the same analyte in human breath. The same chemical system is used in both cases, which is based on the CO 2 (g) enhancement of luminol chemiluminescence in the absence of added oxidant and in the presence of cobalt(II) phthalocyanine complex as rate modifier. A specially designed reaction cell is at the heart of these determinations. The main analytical virtues of these approaches are simplicity, fast response, and a wide linear concentration range amenable to determination (in log-log relationships from about 50 ppm to 800 ppm v/v, and from 1% to 100% v/v CO 2 (g)).