Optical positions, relative to the International Reference Stars catalogue (IRS), and therefore in the system of the FK5, have been obtained for the optical counterparts of 35 extragalactic radio sources south of {delta}~+3deg. Many of these sources are being considered as possible Benchmark objects for the establishment of a quasi-inertial unified radio/optical reference frame. Precision levels better than 0.2" in both coordinates were achieved. A comparison with VLBI radio positions available for these sources is presented. We give evidence for a possible offset in RA between the radio and optical reference frames.