[en] The paper has been written with the aim to determine the initial precondition for neutron spectra unfolding procedure construction depending on a-priori information, utilization of the obtained results, the set of measured reaction rates, their deviations, information and computing resources, etc. The main mathematical and physical problem peculiarities are discussed, the role of initial estimates is determined, unfolding method and error classification is given and, finally, problem definition for neutron spectra unfolding from measured reaction rates is provided. (author)