Accurate determinations of very low concentrations of carbon monoxide are required of samples from pits troubled with spontaneous combustion and those in which diesel locomotives are operating. This method involves the passage of a known volume of the sample first through a train of reagents for purifying and drying and then through a heated tube of iodine pentoxide; if carbon monoxide is present, proportional amounts of iodine and carbon dioxide are formed, either of which may be determined.