Evidences of the pi-stacking of rhodamine B onto poly(sodium 4-styrenesulfonate) are given by H-1 NMR spectroscopy. As a consequence of these pi-pi interactions, changes on the diafiltration and UV-vis absorbance and fluorescence patterns are shown comparing water-soluble polymers containing aromatic rings as poly(sodium 4-styrenesulfonate) and poly(N-methacryloyl-5-aminosalicylic acid) with other polyanions that do not contain aromatic rings as poly(sodium vinylsulfonate) and poly(acrylic acid). (c) 2006 Elsevier Ltd. All rights reserved.