The influence of different borazinic precursors on mesoporous boron nitride synthesis by using a nanocasting process of a mesoporous CMK-3 carbon is presented. Two borazinic precursors, the tri(methylamino)borazine (MAB) and the tri(chloro)borazine (TCB), have been converted to boron nitride (BN) inside the mesopores of a CMK-3 carbon mesoporous template by using thermal or chemical polycondensation processes. Ordered mesoporous boron nitride with a specific surface area around 800 m2/g, a mesoporous volume around 0.6 cm3/g, and a pore-size distribution located at 6 nm in diameter was synthesized by thermal condensation of a molecular MAB precursor. In addition, chemical condensation of TCB led to a disordered mesoporous boron nitride.