[en] Quantitative phase analysis is an important aspect of the quality control of uranium-plutonium mixed oxide and carbide fuels for fast reactors. It has been described how the sintered pellet of uranium-plutonium mixed oxide, consisting of a mixture of solid solutions with varying uranium and plutonium concentrations, is analysed quantitatively for individual mixed crystal fractions. Assuming that the line profile of a pure crystal can be either Gaussian, Cauchy, Error or Arc tangent function, the X-ray diffraction pattern of a mixed oxide pellet is analysed by regression analysis to determine the individual mixed crystal fraction. An absolute method for the quantitative phase analysis of mono-, di- and sesquicarbide and dioxide in the U-Pu-C-O system by X-ray diffraction has been described. Theoretically, the integrated intensity for the different selected lattice planes of the above phases are calculated taking into consideration the crystal structure, Lorentz polarisation factor, multiplicity factor, temperature factor, absorption factor and unit cell volume. For the pure uranium mono-, di-, sesquicarbide and dioxide the calculated intensity ratios between different planes were found comparable with the corresponding measured integrated intensity ratios, thus confirming the basis of calculation. Using these calculated values and experimentally determining the corresponding integrated intensities of the X-ray diffraction pattern any carbide pellet of U, Pu or 'U-Pu' can be quantitatively analysed directly for mono-, di-, sesquicarbide and oxide phases. (author)