tWe report a method for the simultaneous measurement of carbon dioxide (CO2) and carbon monoxide(CO) absolute concentrations and temperature using fiber laser intracavity absorption spectroscopy (FLI-CAS). FLICAS uses an erbium-doped broadband laser capable of being tuned in the range 6350–6450 cm−1.The range 6390–6410 cm−1, which is emitted by the laser without tuning, allows for simultaneous obser-vation of the CO, CO2, and methane (CH4) spectra. Truly simultaneous observation of these spectra ispossible during one laser pulse with duration as short as 3.7 s. The sensitivity of the CO measurementis about 20 times higher than that of CO2, a feature which is advantageous for several applications.The concentration of CO can be measured with a sensitivity of 400 ppm at room temperature, and thissensitivity can be improved by increasing the generation time. The experiments were performed in atemperature and flow-controlled cell in order to evaluate the accuracy of the concentration and temper-ature measurements from 296 to 1200 K. The measured spectra of CO and CO2are well described by theHITRAN database, allowing for accurate and simultaneous determination of CO, CO2and temperature.The temperature is evaluated using the CO2spectrum, which includes many “hot” transitions. The CH4HITRAN data are deficient at high temperatures, so this method only allows for the evaluation of methaneconcentrations at room temperature.