Hydrophobically modified alkali-soluble acrylic latexes were prepared using varying level of SEM-25, triphenylethyl phenoxy polyoxyethylene ether methacrylate, a functional monomer bearing a long segment of ethylene oxide and a hydrophobic tetraphenyl group. Sharp increases in viscosity and light transmittance of the diluted associative thickener latexes were observed with ascendant pH starting from 7. And with further increase in the pH, viscosity of the thickener latexes rapidly fell down, while the light transmittance was kept relatively constant. It was revealed that a critical content of SEM-25 in the thickener copolymer, 5% in the present case, was required in order for the associative thickener to outperform the SEM-25 free counterpart. When used in thickening a latex of P(St-BA) (Poly(styrene-Butyl acrylate)) copolymer using a thickener copolymer with 12 wt% of SEM-25, addition of 0.34 wt% of the thickener polymer relative to P(St-BA) was necessary to have a higher viscosity than the same latex thickened using the thickener free of SEM-25. It seemed that incorporation of 0.275 mol% of SEM-25 was indispensable for the associative thickener to exceed the performance of conventional thickener.