The Los Alamos National Laboratory Monte Carlo MCNP code is applied to critical experiments performed at the RBMK critical facility of the Russian Research Center “Kurchatov Institute,” Moscow. The validation investigations are completed by whole-core criticality calculations of experiments at the Smolensk Unit 3 nuclear power plant as part of the start-up procedure. The geometric model exploits the powerful capabilities of MCNP by precise representation of the fuel and different types of nonfuel channels, which add up to a detailed model of the criticalfacility and the RBMK core. Continuous-energy cross-section tables are taken from the ENDF/B-IV and ENDF/B-VI libraries. As the most important uncertainty inherent to the experimental setup, the concentration of impurity isotopes in the graphite moderator is identified. Within the resulting error limits, the keffand the void effect are well reproduced with both cross-section libraries.