An electrically heated laboratory scale fluidized bed reactor was used for measurement of carbon dioxide concentrations from the devolatilization of bituminous coal under conditions pertinent to a large scale atmospheric pressure fluidized bed combustor (AFBC). Feeding a few particles into the bed, which contained 1 or 2 g of char, produced an increase in CO2 concentration. The quantity of CO2 released from the fluidized bed was found to be a function of fluidizing solid material, as well as the bed temperature and oxygen concentration in the fluidizing gas. Silica sand, magnesium oxide, calcium sulfate and partially sulfated Reed lime from a larger continuously fed AFBC (used bed material) were utilized as fluidizing solid particles. CO2 concentration profiles in the effluent gas were integrated and converted to the weight fractions of coal particles that created disturbance in the CO2 level. For the silica sand bed, under the conditions of pyrolysis in the absence of oxygen, the quantity of CO2 evolved from the bed was increased from 1 to 3 wt% coal when the bed temperature was varied from 950 to 1150 K. For partially sulfated Reed lime, under similar conditions, the CO2 quantity increased from 2.4 to 23.4 wt% coal. Magnesium oxide particles had shown similar behaviour to silica sand solids, while the calcium sulfate bed had released a larger CO2 quantity, like the used bed material. Thus, under similar conditions, the activity of the various fluidizing materials can be compared by CO2 measurement.