In the present work the performance of nanofiltration in the recovery of caustic cleaning solutions, coming from the dairy industry, with high COD (23.0 and 12.0 kg/m(3)) was investigated. Surprisingly, the permeate flux was higher for the caustic solution with the significantly higher COD. This was on the average about 1.4 x 10(-5) m(3)/m(2)/s, whereas in the second case the filtrate flux was about 0.9 x 10(-5) m(3)/m(2)/s. In the permeate of the more highly polluted solution a COD of 0.5 kg/m(3) was measured. The COD of the second permeate stream was 1.5 kg/m(3). The noticeable lower contamination could be attributed above all to the higher flux. The degree of pollution (COD) was therefore not the principal determining factor of the permeate quality. While the flux rose linearly with increasing pressure, measurements showed in both cases that the cross-flow velocity had no influence on the performance of the nanofiltration. This could be explained by the pulsation effect of the feed (circulation) pump used. The quantity of the recycled base was dependent mainly upon the recovered volumetric yield (practically no NaOH retention), which was around 90%.