[en] For characterisation of the neutron fluence shape in a reactor, two parameters are relevant: (1) the thermal-to-epithermal fluence rate ratio (f = φ_th/φ_e), and (2) the parameter α in the 1/E^1+α epithermal spectrum representation. Both parameters are determined experimentally, thus introducing errors in the results. The influence and propagation of these errors can be also calculated. These characteristics were experimentally determined for some irradiation channels in the TRIGA Mark II reactor of the Jozef Stefan Institute, and the experimental errors introduced compared to calculated ones. A comparison between the experimentally obtained uncertainties and the calculated ones is outlined and the influence of particular parameters on error propagation when determining α and f are elucidated. (author)