The physical behavior of the mockup burnable poison rods, containing 0.5, 1.0, and 2.0 wt% Gd2O3 in natural uranium oxide pellets, has been studied in critical experiments. Specifically, the reactivity effects, thermal-neutron density distributions, and overall temperature coefficient of reactivity were of interest when related to the poison effect. It was shown that the poison reactivity effect did not change greatly when the Gd2O3 content of the fuel pellets was >0.5 wt%. The poison also made the temperature coefficient of reactivity more negative. Some analytical methods are given, and the calculated and experimental results are compared.