Abstract: The ion-associate formation between tetraiodofluorscein and benzalkonium in the presence of quinine was investigated. The tertiary ion-associate is found to have a composition of quinine: benzalkonium: tetraiodofluorescein (1: 1 : 1) with molar absorptivity in 1,2-dichloroethane of 5.8×10⁴ 1·mol^-1·cm^-1 (λ_max=545 nm). Beer's law is obeyed over the benzalkonium concentration range of 2×10^-7～4×10(-6)M. A highly sensitive method is suggested for the benzalkonium salt determination. Many other quarternary ammonium species, amines and alkaloids do not interfere. Sixteen other acidic dye—quinine—benzalkonium extraction-spectrephotometric systems were also studied. Fluoresceins were found to show higher molar absorptivities than that of phenolsulfonphthaleins of similar structure.