Solid fuels as a renewable source of energy are now increasingly in use as fuel in bio plants. During their combustion, ash is produced which needs to be further used as raw material or disposed of at landfills. The composition of ash is very important because the content of certain oxides significantly influences the possible further use of ash. Determination of ash composition includes determination of oxide content: Na2O, K2O, CaO, MgO, Al2O3, Fe2O3 and SiO2. For their determination, the most commonly used is EDXRF spectrometer as an excellent analytical technique for fast and precise determination of a large number of elements in complex samples. The problem with the development of the EDXRF method for analyzing the composition of ash biomass lies in providing a sufficient number of reference materials for method calibration. As there are no reference biomass materials available on the market, one way is to determine the composition of ash by another analytical technique (ICP-OES) and apply these results as reference values for developing the EDXRF spectrometer method. The experimental work will include digestion of biomass ashes by microwave digestion and determination of the main elements of the ICP-OES. The determination method will be tested with the certified reference material NIST SRM 2690 Coal Fly ash. Based on the results obtained, the same ash samples will be used to calibrate the method for determining the composition of ash on EDXRF. After the method is developed, validation will be performed. The method for determination of ash composition will be applied on real samples of ash from the bio power plant.