In-core thermionic reactors (ITR) contain in-core materials which are very unusual in reactor physics. For this reason and due to the compact and very heterogeneous structure of such reactors, critical experiments for an ITR prototype were carried out Different core configurations with measurements of critical masses, reactivity worths, fluxes, and power distributions, also provided the opportunity to test methods of computation, special nuclear data, and measuring methods on a more general basis.