Planar pyridyl N-oxides are encapsulated in mono-metallic Pd(II)/Pt(II)-cages based on a tetra-pyridyl calix[4]pyrrole ligand. The exchange dynamics of the cage complexes are slow on both the NMR chemical shift and EXSY timescales, but encapsulation of the guests by the cages is fast on the human timescale. A “french doors” mechanism, involving the rotation of the meso-phenyl walls of the cages, allows the passage of the planar guests. The encapsulation of quinuclidine N-oxide, a sterically more demanding guest, is slower than pyridyl N-oxides in the Pd(II)-cage, and does not take place in the Pt(II)-counterpart. A modification of the encapsulation mechanism for the quinuclidine N-oxide is postulated that requires the partial dissociation of the Pd(II)-cage. The substrate binding selectivity featured by the cages is related to their different guest uptake/release mechanisms.