Polarization microscope studies on single crystal samples of Pb(Fe1/2Nb1/2)O3 (PFN) have been made. Thin platelets (<60 μ) show large, homogeneous ferroelectrics domains, and optical characterization shows that PFN is trigonal at room temperature and optically negative. Birefringence measurements between 4.2 and 400 K reveal that the transition to the cubic structure takes place at 383 K, in accordance with earlier work, but they also locate a previously undetected structural modification at 354 K, accompanied simultaneously by a reorientation of the index ellipsoid; both transitions manifest themselves by endothermic reactions on heating in DTA experiments. The new phase is very likely monoclinic.