The cloud points (CP) of aqueous solutions of two ethylene oxide–propylene oxide–ethylene oxide tri-block copolymers (EO)10(PO)16(EO)10 and (EO)1(PO)17(EO)1 were measured in the presence of additives, such as salts, alkalis, acids, polymer and ionic surfactant. The results obtained in this study indicated that salts decrease the CP of the two tri-block copolymers in the order of NaCl>CaCl2>MgCl2 and Na3PO4>Na2SO4>NaCl. Alkalis caused a sharp drop of CP of the two tri-block copolymers. Acids increased the CP. Water soluble polymer such as polyvinylpyrrolidone (PVP), hydrolysis polyacrylamide (HPAM) and cation-polyacryamide (cation-PAM) decreased the CP. Ionic surfactants such as sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) increased the CP. The extent of lowering depends on the structure and concentration of additives and the number of ethylene oxide group of tri-block copolymers.