This paper fully describes a whole-core benchmark problem based on a small two-loop pressurized water reactor composed of UO2 and MOX fuel assemblies. The specification includes heterogeneity at both the assembly and the core levels. The geometry and material compositions are fully described, and multigroup material cross-section libraries are provided in the two-group, four-group, and eight-group formats. Detailed Monte Carlo reference solutions including core eigenvalue, assembly-averaged fission distribution, and selected fuel pin fission density distributions are presented for benchmarking diffusion and transport methods. Three different core configurations are presented in this paper, namely, all-rods-out, all-rods-in, and some-rods-in.