At the PWR Paks diluted low-level radioactive waste water (LLW) contains the long-lived Co-60 isotope in EDTA complex form, which has no simple separation procedure. In this research nanofiltration (NF) was studied for selective removal of the cobalt(III) EDTA complex from a drain waste wafer model solution, which contains mainly sodium borate at an alkaline pH. A suitable NF membrane was chosen to separate the cobalt complex from the borate solution. The NF experiments were performed at constant temperature (25 degrees C) and pressure range 1-10 bar. The cobalt complex and the borate ion rejection (R) as well as the permeate flux of the membrane was investigated as a function of pH. The rejection of the cobalt(III) EDTA complex ion and especially the borate were strongly pH dependent. The rejection of the complex ion and the borate was increased at alkaline pH (at pH 8, R = 73%; at pH 11.5, R = 96% for the cobalt complex; at pH 8, R = 7%; at pH 11.5, R = 59% for borate). NF seems to be a suitable separation method for the removal of the Co(III) EDTA complex from nuclear power plant waste streams. The removal of the cobalt complex ion from an alkaline borate solution by NF is possible in two ways: at slightly alkaline pH by a two-step separation, or at a more alkaline pH (pH > 9.5) by a one-step separation.