The thermal evolution of the crystal structure of PrSr3Co1.5Fe1.5O10-d, a member of the n = 3 family of Ruddlesden-Popper compounds, has been studied by means of variable temperature synchrotron X-ray powder diffraction combined with room temperature neutron diffraction studies. This structure takes the ideal tetragonal I4/mmm (n degrees 139) space group. The possibility of symmetry lowering to the Pbca (n degrees 61) space group by slight tilting and rotation of the oxygen atoms around the octahedral B site is discussed. The non linear thermal expansion of the compound in air is caused by a chemical expansion accompanying the loss of oxygen that comes in addition to normal thermal expansion. A mechanism describing the creation of oxygen vacancies is proposed.