When a sample is fused with a flux to produce a glass button for X-ray fluorescence analysis, some of the constituents are lost by decomposition (H2O, CO2) or by evaporation (Br, Cl, S) . Therefore the glass button is smaller than if no “Loss on Ignition” had occurred, and the X-ray intensities appear stronger. It is shown that “L.O.I.” can be handled as a regular element and that corrections can be made by means of influence coefficients. Since “L.O.I.” cannot be measured by X-ray fluorescence, one should normally know the “L.O.I.” to make the appropriate corrections, but under certain conditions, X-ray intensities yield both composition and “L.O.I.” as shown by Tertian and as confirmed by our experiments.