The disappearance of carbon monoxide in mines is vitally linked with the study of underground fires it is not yet known whether the cause for such disappearance is chemical or bacterial. The authors’ investigation tends to indicate that bacteria play a major part in CO disappearance. Two coals of different ranks were studied and the rate of disappearance of CO was found to be practically the same for both. It was also observed that CO disappears faster over wet coals than over dry coals and a certain amount of time is always required before CO starts disappearing