A portable cavity ringdown spectroscopy (CRDS) apparatus was used to detect effluents from small test fires in the Fire Emulator/Detector Evaluator (FE/DE) and a small room in the Building Fire and Research Laboratory at the National Institute of Standards and Technology (NIST). The output from two lasers is combined to detect four combustion gases, CO, ${\mathrm{CO}}_2$ , HCN, and ${\mathrm{C}}_2{\mathrm{H}}_2$ , near simultaneously using CRDS. The goal of this work was to demonstrate the feasibility of using a CRDS sensor as a fire detector. Fire effluents were extracted from several test facilities and measurements of CO, ${\mathrm{CO}}_2$ , HCN, and ${\mathrm{C}}_2{\mathrm{H}}_2$ were obtained every 25–30 s. In the FE/DE test, peak concentrations of the gases from smoldering paper were $420\mathrm{parts}\mathrm{in}{10}^6(\mathrm{ppm})$ CO, $1600\mathrm{ppm}$ ${\mathrm{CO}}_2$ , $530\mathrm{parts}\mathrm{in}{10}^9(\mathrm{ppb})$ HCN, and $440\mathrm{ppb}$ ${\mathrm{C}}_2{\mathrm{H}}_2$ . Peak gas concentrations from the small room were $270\mathrm{ppm}$ CO, $2100\mathrm{ppm}$ ${\mathrm{CO}}_2$ , and $310\mathrm{ppb}$ ${\mathrm{C}}_2{\mathrm{H}}_2$ .