The orthorhombic ferroelastic domains of K3Fe5F15, with prototypic tetragonal W bronze (TTB) structure and mixed valence 3Fe2+/2Fe3+, were studied at R. T. in polarized light on single crystals grown by high temp. soln. growth with subsequent quenching from 973 K. The birefringent ferroelastic domains exhibit a remarkable linear dichroism. The principal refractive indexes and the principal extinction coeffs. were measured along the crystallog. axes, identified by x-ray measurements on a single domain. The correlation between the refractive indexes, the extinction coeffs. and the cell parameters was established. The change of orientation of the vibration ellipse of the transmitted light due to dichroism was obsd.