Single-crystal X-ray diffraction has been used to characterise the structural properties of a CaIrO₃ orthorhombic perovskite-type structure under ambient and high pressures. It has been found that the structure of this compound is more distorted than other CaO perovskites. Structural determination at high pressure also shows a peculiar compression behaviour that is different from the other CaO perovskites. This explains why this material is much stiffer than expected.