The Segment-eNRTL model has been applied in order to predict mean ionic activity coefficients for complex polymer/water mixtures. Special attention is paid to the segmentation approach which is nontrivial for complex polymers and therefore the crucial point for the model performance. Reasonable segmentation approaches are elaborated and discussed in detail. The segment number is calculated with a new approach recently published. Solubilities of KH2PO4 in polyether-polyol/water mixtures have been experimentally studied. The best agreement between experimental data and model predictions is obtained by using segments of one type and an extended temperature relationship for the segment/ion short-range interactions. Moreover, theoretical evidence for these results are provided.