[en] The k₀-standardization method of neutron activation analysis (k₀-NAA) was launched in the 1970s and since then continuously developed. Nowadays, k₀-NAA became widespread as a practical analytical tool used to analyse different sample matrices. At the Jozef Stefan Institute (IJS), the KAYZERO/SOLCOI software package has been introduced for data processing after extensive testing and comparison with other available programs. In the process of validation of the software a suite of natural matrix reference materials (RMs) were used. Five certified reference materials (CRMs) from the Institute for Reference Materials and Measurements (IRMM), two standard reference materials (SRMs) from the National Institute of Standards and Technology (NIST), three RMs from the International Atomic Energy Agency (IAEA) and one RM from IJS were analysed. Altogether, results for ten elements in inorganic matrices and twenty-one elements in organic matrices, obtained by k₀-instrumental neutron activation analysis (k₀-INAA), were compared to certified values. The results obtained show good agreement with certified or assigned values except for Fe and U in inorganic matrices, and Al and Cr in organic matrices. (author)