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Octahedral tilting and polyhedral distortions of several Ca-oxides having the perovskite structure, including new data on CaIrO3, have been evaluated using both the unit-cell lattice parameters and the amplitude of the oxygen and Ca displacements from their hypothetical positions in an ideal cubic perovskite structure. The two types of calculation differ significantly, indicating that polyhedral distortions cannot be neglected for these perovskites. Whereas most of the Ca-oxide perovskites follow linear trends in terms of polyhedral volumes and tilting, those having a Pt-group element (i.e. Ir, Ru and Rh) occupying the octahedral site clearly deviate from such trends. In particular, CaIrO3, CaRuO3 and CaRhO3 appear to have much larger tilting and distortion, likely causing the structure to be stiffer than expected from the analysis of the bulk modulus variation of other Ca-oxide perovskites. Structural refinements of intensity data collected at high-pressure for a single crystal of CaIrO3 perovskite, also show that the IrO6 octahedra and of CaO12 polyhedra have similar compressibility, contrary to other Ca-oxide perovskites for which the Ca polyhedron is more compressible than the octahedral site. This gives rise to an octahedral tilting that is independent of pressure at least up to 10 GPa. CaIrO3, CaRhO3 and CaRuO3 perovskites have been found to undergo a phase transformation to an orthorhombic Cmcm post-perovskite phase (CaIrO3-type structure) which is stable or can be quenched at room pressure, suggesting that a large octahedral tilting, a stiffer structure and likely a similar compressibility of the two perovskite cation sites may be a requisite for stabilising the orthorhombic post-perovskite phase.