The properties of thermally sprayed coatings significantly depend on the alloy composition and the adjusted process parameters. In addition to the powder certificate it may be useful to analyse the chemical composition of the sprayed powder during the spraying process itself. The principle of composition analysis is similar to the chemical analysis in an ICP plasma but the boundary conditions are more complex because the sprayed powder should not be completely evaporated in a thermal spray process. Nevertheless all thermal spraying processes lead to a certain evaporation of the species and to excitation of atomic states. The transition into the ground state occurs under emission of characteristic lines. The intensity of these lines is influenced by the plasma temperature, the particle temperature, the temperature dependent evaporation rate of the alloying elements and the powder feed rate. In consideration of the boundary conditions and the information from a detailed analysis of the emitted spectra the lines can be used to quantify the chemical composition of the sprayed alloys online. The theory of the principle for on-line analysing the chemical composition will be deduced and the first experimental validation will be presented.