Several recent publications have shown that it is possible to use Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) in the direct analysis of elemental pollutants in flue gases produced by industrial processes. The limits of detection observed are lower than the threshold limit values in the gaseous effluents of incineration and other industrial plants. The two crucial points in precision and accuracy are calibration and gas composition. Calibration is carried out by means of a standard aerosol produced using a standard solution, an ultrasonic nebulizer, and a desolvation unit. Precise control of the water content results in a reduction in the errors to below 20 % when the gas composition is stable. Gas composition variations can induce important changes in emission intensities and errors in concentration measurements. In order to limit errors, this gas composition has to be controlled and the calibration carried out with a gas of the same composition as the gas in which heavy metals are to be analysed. It is also possible to choose operating conditions not sensitive to possible changes, but it results in degradation of detection limits. This technique has been tested on two processes. The first one was a pilot plant for fly ash vitrification, the second one a coal power plant. These tests confirm that the control of the gas composition is a key point, which has not been sufficiently considered until now.