Ultrahigh resolution synchrotron x-ray powder diffraction shows up two structural transitions in the layered cobaltite YBaCo2O5.50. The room-temperature orthorhombic structure transforms, on cooling below TMI≃295 K, to monoclinic coinciding with the apparition of a ferromagnetic moment and the occurrence of the metal to insulator transition. On further cooling, the ferromagnetic component disappears and orthorhombic symmetry is recovered.