[en] For studies in fuel rod internal chemistry special experimental procedures are needed in order to obtain results which are of real value for the improvement of knowledge in this field. Some investigation techniques are described which have been developed in KWU radiochemistry and fuel development laboratories. A part of them is directed towards the determination of migration and distribution of fission products (especially Cs and I) in LWR fuel. With heating experiments of mock-up materials radiotracers or short-term irradiations are used and fission product distribution is measured by micro γ scanning. From spent fuels microsamples are taken at definite locations; analysis is performed by γ spectrometry and neutron activation analysis. Fission product escape from defective fuel rods during power reactor operation is studied by radiochemical coolant analysis. Finally, O/M ratio of spent oxide fuel is analyzed by controlled-potential coulometry. (orig.)