Using the substitution method combined with the pulsed neutron technique, coolant void reactivities of PuO2-UO2 fuel lattices in pressure-tube-type heavy water reactors have been determined as functions of PuO2 enrichment in PuO2-UO2 (0.54 and 0.87 wt%), fissile content of plutonium (91 and 75% fissile plutonium), lattice pitch (Vm/Vf: 7.4 and 9.9), and coolant void fraction (0, 30, 70, 87, and 100%). The reference loading of 1.2 wt% enriched UO2 clusters was progressively replaced by PuO2-UO2 test clusters. The void reactivities were obtained from Simmons and King’s formula in which correction was made for a change of the prompt generation time. As decay constants can be maintained invariable due to substitution, buckling differences were analyzed by the first-order perturbation method, on the assumption that lattices are homogeneous and no difference in diffusion coefficients exists between the two lattices. Void reactivities of test lattices were determined with an accuracy of ∼10% when the minimum number of test fuel clusters was ∼5% of the total. The void reactivity shifted farther to the negative side as the proportion of fissile plutonium was increasingly in the PuO2-UO2 fuel of the same enrichment of plutonium.