The rejections of four un-dissociated organic compounds reached negative values in the presence of increasing salt concentrations and salt valances. Concentration polarisation did not offer an explanation for these results. An inspection of the rejection curves for both the organic solute and salt found a pattern in the rejection behaviour. Salt rejection was seen to initially increase and then decrease as a function of its own concentration. This decrease in salt rejection coincided with a marked slowing in the rate at which the solute rejection was changing, reinforcing the link between the presence of salt and rejection of the organic solute. It is proposed that a small charge property of the organic solute is central to this process and not the classic "salting out" effect. Current modelling theory was considered and then modified such that the valance term became a non-integer value and described the polarity of the organic molecule. The polarity was ascribed a value related to valence that ranged from 0.0045 to 0.025 for the four organic compounds investigated. This contrasts with typical values for valances of ions that are integers which depend on the dissociation of the inorganic compound. (C) 2009 Elsevier B.V. All rights reserved.