The retention of a single salt, a single sugar and combinations of salt and sugar have been studied. The retention of the salt was dependent on the salt concentration. The retention of the sugar was neither dependent on the sugar concentration, nor on the crossflow velocity for sugar concentrations up to 90 g/l and a Reynolds number from 2200 to 6600. The observed sugar retentions were very high and could have hidden influences on the transport phenomena. When both salt and sugar were present, the salt retention was dependent on both salt and sugar concentration. An increased sugar concentration gave a decreased salt retention. This could be explained quantitatively with the increased viscosity near the membrane surface.