[en] In the centre of the STEK oscillation cores (STEK-500, -1000, -2000, -3000 and -4000) fission rate measurements of ²³⁸U and ²³⁵U were performed with the help of depleted and enriched uranium metal activation foils and with a double absolute fission chamber. The absolute fission rate of ²³⁵U is needed for the normalization of the reactivity worths of the fission product samples which have been measured with the reactor oscillator. In the clean STEK (i.e. with the oscillator element replaced by a standard element) cores a set of fission rate measurements was performed with the absolute fission chamber for ²³³U, ²³⁵U, ²³⁸U, ²³⁷Np, ²³⁹Pu, ²⁴⁰Pu and ²⁴²Pu. From these reaction rates the spectrum indices with respect to ²³⁵U are calculated. These measured spectrum indices can be converted to spectrum indices which 'would have been measured' in the oscillation core by means of calculated correction factors. These spectrum indices in the centre of the oscillation cores are compared to the corresponding calculated values. In the oscillation cores axial fission rate traverses of ²³⁸U and ²³⁵U have been measured with the double absolute fission counter. Further, in the clean cores axial and radial fission rate traverses of ²³⁸U and ²³⁵U have been measured using depleted and enriched uranium activation foils. From these fission rate traverses the traverse of the spectrum index sigma^-28sub(f)/sigma^-25sub(f) are calculated. The results of these traverse measurements show that the flux spectrum is nearly constant in a cylinder in the centre of the core with a radius of about 12 cm and reaching from 10 cm under to 10 cm above the mid fuel plane