The effects on coolant void reactivity from soluble poison and from control rods inserted into the moderator of a light-water-cooled pressure-tube-type heavy water reactor (HWR) are studied in experiments and theoretical analyses. The soluble neutron absorber is 10B burnable poison uniformly dissolved in the moderator, and the boron carbide control rods are inserted into the moderator vertically between fuel channels. The reactivity caused by the increased void fraction is measured in the deuterium critical assembly (DCA). The void reactivity becomes less negative with the soluble neutron absorber, and the change is nearly proportional to the concentration of poison. The void reactivity is not as dependent on the number of control rods inserted, and the incremental positive shift lessens with increasing control rod worth. Experimental and calculated (WIMS-D4 code) results agree within ±1 $., The effects on void reactivity caused by the neutron absorbers are investigated by perturbation analysis. Neutrons are easily thermalized by light water in the pressure tube at lower void fractions, and some diffuse into the heavy water moderator. More thermal neutrons are absorbed in the heavy water in the presence of an absorber than with a higher void fraction.