One of the most suitable methods for analytical on-line determination of actinides in solutions of reprocessing plants is X-ray fluorescence analysis. For stability and for economy of space radionuclides are preferable as excitation sources. The excitation and the measurement of the K-lines has a number of advantages in comparison to the L-lines: the fluorescence yield is higher, the penetration is better, and the resolution is better. To lower the background due to scattering radionuclides with -energies of about 300 keV are preferable.75Se,113Sn,125Sb,131I,133Ba,192Ir,203Hg, and231Pa have been proposed,133Ba was used in this work. The following arrangement was built up: a HPGe-detector, two133Ba sources in 45o position, detector and source collimators made from tin and a sample tube diameter of 130 mm with technical glass or stainless steel of up to 6 mm thickness as tubing material. The measuring time was 10 min. Using this arrangement the calibration curve for uranium was measured in the range from 5 mg/l to 400 g/l. In mixtures of Th and U interferences were observed and corrected for. Samples of Np and Pu were also measured. Determination of Th, U, Np and Pu in mixtures in absence and in presence of other radionuclides is discussed.