Abstract: A method and an apparatus for measuring and controlling the efficiency of a combustion whereby, ash samples are drawn at predetermined time intervals from a region of a combustion plant, each drawn sample is set in an exhausted reaction cell, combustion reaction gas is introduced under controlled pressure, a superficial layer of the sample is heated to the carbon combustion temperature by a CO.sub.2 laser beam, the reaction gas is drawn from the cell and the amount of carbon dioxide produced by the carbon combustion is measured in a calibrated detector. The amount of unburnt carbon contained in the ashes is determined based on a preceding calibration carried out on ashes of known carbon content.