Sensitive and selective spectrophotometric methods are proposed for the determination of chlorhexidine, benzethonium and benzalkonium, which are useful and popular disinfectants in the hospital, by solvent extraction based on ion pair formation. Chlorhexidine reacted with tetrabromophenolphthalein ethyl ester (TBPE) to form a 1 : 2 red violet ion associate (λ_max=570 nm) and its molar absorptivity was 6.77×10⁴l mol^-1 cm^-1. The determination limit was 0.058 μg/ml. This method was more sensitive than other associates with TBPE and applied for the determination of trace amounts of chlorhexidine in the hospital disinfectant waste water. Benzethonium and benzalkonium(bulky quaternary ammonium disinfectants) were selectively determined by extraction based on ternary ion pair formation among quinine, Bromophenol Blue and quaternary ammonium ions. Optimum pH range was 6.57.1. The molar absorptivity of benzethonium as sociate was 5.04×10⁴l mol^-1 cm^-1 and that of benzalkonium was 4.85×10⁴ l mol ^-1cm^-1 and relative standard deviations were about 1.5%. λ_max was at 610 nm. This method is also available for the determination of quaternary ammonium disinfectants in waste water.