A method of determining the concentration of a species in a portion of a furnace atmosphere is described. The method comprises the steps of measuring first, second and third intensities of electromagnetic radiation in the furnace at first, second and third wavelengths respectively. The third wavelength is selected to be representative of absorption of electromagnetic radiation by the species. A fourth intensity of electromagnetic radiation is calculated, being an estimate of the intensity of electromagnetic radiation in the furnace at the third wavelength absent any absorbing species in the furnace atmosphere. The third intensity and the fourth intensities are used to determine a parameter that is proportional to the concentration of absorbing species in the portion of the furnace atmosphere. Apparatus for carrying out the method is also described.