The coagulation behaviour of the new coagulant agent polyaluminium silicate chloride (PASiC) was investigated in this study. The main purpose was the examination of several possible derivatives of polyaluminium silicate chloride, based on different basicity (OH/AI molar ratio), silica content (Al/Si molar ratio) and preparation method (co-polymerization or composite polymerization) to evaluate the respective coagulation behaviour of them. Moreover, a systematic study was conducted to define the optimum values of aforementioned major parameters, in order to produce ail improved product, in comparison with the commonly applied polyaluminium chloride. Overall, 32 silica-based coagulant samples were prepared with different OH/Al (1-2.5), or Al/Si (5-20) molar ratios and preparation methods. The coagulation performance of PASiC products were evaluated for the treatment of contaminated tap water (in terms of turbidity and of NOM removal, as well as of residual Al concentrations and of zeta-potential measurements). Also, they were examined for the tertiary treatment of municipal wastewater (mainly for phosphates removal). Additionally, the new products were compared with the laboratory prepared PACI, with alum (i.e. Al-2(SO4)(3)-18H(2)O), as well as with commercially available PACI samples. These experiments were completed with the study of coagulation kinetics by using the Photometric Dispersion Analyzer (PDA), in order to compare the respective floc growth rates. Overall, the obtained results suggest that in order to produce a silica-based polyaluminium coagulant with improved coagulation properties, the basicity (OH/Al ratio) should be between 1.5 and 2.0, the silica content (Al/Si molar ratio) between 10 and 45 and should be prepared preferable with the co-polymerization technique. However, attention has to be given in the specific application of these products, as in the case of tertiary wastewater treatment (phosphates removal) more efficient seem to be the silica-based coagulant with lower basicity (i.e. OH/Al (C) 2008 Elsevier B.V. All rights reserved.