The aim of this work was to investigate the properties of Sicopion membranes: an ultrathin (≈20？μm) composite cation-exchange membrane (CEM) made from sulphonated poly(ether-ether-ketone) (SPEEK) containing different levels of sulphonic-functionalized silica particles (SFSPs). Sicopion membranes were conditioned according to the French Normalization Association procedure, consisting in a series of acid and alkaline washes, and their electrodialytic characteristics were compared to an existent commercial food-grade membrane (CMX-SB). Electrical conductivity of Sicopion membranes was higher than that of CMX-SB membranes (9.92 versus 6.98？mS/cm), as well as their water content (34.0 versus 27.6%). As the SFSP level was reduced, the ion-exchange capacity (IEC) of Sicopion membranes increased. Concerning their electrodialytic performances, Sicopion membranes presented a lower demineralization rate than CMX-SB membranes (35.9 versus 45.5%), due to an OH？ leakage through the pores created by dislodging the SFSP particles during the conditioning procedure. 1. Introduction Sicopion is a novel type of membranes made of a composite material combining sulphonic-functionalized silica particles (SFSP) and sulphonated poly(ether-ether-ketone) (SPEEK). In this composite material, the functionalization of silica particles is accomplished by covalently grafting sulphonic acid groups [1]. In the case of SPEEK, concentrated H2SO4 is used to sulphonate, also by covalent bonding, an aromatic chain of poly(ether-ether-ketone) (PEEK) [1, 2]. Once the organic and the inorganic phases prepared, they are mixed in specific proportions according to the application requirements. The resulting composite material is then prepared in a membrane form. Among the factors affecting the membrane properties there is the sulphonation degree of the components and the type and amount of SFSP used. A PEEK should be partially sulphonated to decrease its hydrophobicity and produce an ion-exchanging material, but high degrees of sulphonation are to be avoided, since the polymer would then become water soluble, which is the case of 100% SPEEK [1, 2]. The amount of SFSPs, as well as their size, determines the formation of a continuous hydrophilic phase and the mechanical properties of the material. A high content of inorganic particles (i.e., SFSP) produces higher membrane hydration and water retention capacity but, in contrast, yields a brittle membrane with lower tear resistance [1]. These materials impart to the membrane a highly functional ion-exchanging structure without inert fillers. At the
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