The objective of this study was to investigate the influence of aluminium coagulants basicity on the minimalisation of undesirable effects of coagulation such as decreasing pH and alkalinity, thereby water corrosivity. The experiments involved three types of surface water samples: from the Odra and Oława Rivers and a mountain stream, which differed in physico-chemical composition (especially in pH values and alkalinity). Alum (ALS) and polyaluminium chlorides (PACI) characterized by various basicity (r, %) were used as coagulants. The experimental results showed that decrease in water corrosivity depended on the basicity and dose of coagulant, pH and temperature of water prior to coagulation. With the increment of coagulation basic i ty the chemical stability of treated water improved and coagulants PAX-XL 3 and PAX-XL6 l were the best. Using of polyaluminium chlorides instead of alum decreased not only undesirable effects of coagulation but improved also the efficiency of water treatment.