Mesoporous Poly(benzimidazole) with well-defined porosity and pore diameter of 11 nm was synthesized via a hard templating approach using silica nanoparticles as template. The effect of the template concentration as well as the influence of the cross-linking density on the resulting pore structure was investigated. The samples were characterized by small-angle X-ray scattering and nitrogen sorption, revealing a linear increase of the porosity and surface area with increasing content of the template up to a critical concentration. Exceeding this concentration leads to a breakdown of some pores due to the emerging high interfacial energy. The effect of the cross-linking density was shown to be nonlinear. Samples with more than 20 mol % cross-linker revealed perfect replication of the template, while a lower cross-linking density leads to a breakdown of some pores.