Material bucklings have been determined as functions of 235U enrichment in UO2 (0.7, 1.2, and 1.5 wt% 235U), PuO2 enrichment in PuO2-UO2 (0.54 and 0.87 wt% PuO2), fissile content of plutonium (91 and 75% Pu-fissile), lattice pitch (Vmod/Vfuel: 7.4 and 9.9), and coolant void fraction. The reference loading of 1.2 wt% 235U-enriched UO2 clusters was progressively replaced by the test clusters. Buckling differences resulting from the substitutions were analyzed by the new second-order (iterative) perturbation method, on the assumption that neutron diffusion is isotropic and that no difference in diffusion coefficients exists between the two lattices. This analysis takes into account the effect of distortion in radial neutron flux distribution in the substituted core without any iterative correction procedure that is usually adopted in the first-order perturbation method. Also, it is not necessary in the case of the present analysis to introduce any usual intermediate region for taking into account the effect of spectrum mismatch between the two lattices. The material buckling differences between the test and reference lattices, which are in the range of −10.2 to 9.1 m−2, were determined within 3% of uncertainty.