Quantitative in situ attenuated total internal reflection Fourier transform infrared (AIR FTIR) spectroscopy has been used to study the isotherm of poly(sodium 4-styrene sulfonate), PSS, adsorption to a TiO2 surface in aqueous solution at a pH of 3.5. The effect of adding surfactant cetylpyridinium bromide monohydrate (CPBM) on the adsorption isotherm of PSS was investigated at CPBM concentrations of 3.60 x 10(-7), 1.02 x 10(-5), and 1.04 x 10(-4) M. The use of in situ ATR FTIR allowed for the calculation of the concentration of both PSS and CPBM at the TiO2/water interface over the entire course of all experiments. It was found that the addition of a small amount of CPBM, 3.60 x 10(-7) M, to PSS solutions resulted in 23 +/- 3% less PSS accumulating at the TiO2/water interface compared to isotherm studies with no CPBM present. The mole ratio of CPBM to PSS varies from 4 +/- 1 to 1 to 20 +/- 4 to 1 in a stepwise manner as the solution concentration of PSS is increased for solutions with a CPBM concentration of 3.60 x 10(-7). The addition of CPBM at concentrations of 1.02 x 10(-5) and 1.04 x 10(-4) M showed distinct differences in the behavior of the PSS isotherm, but at the highest solution PSS concentrations, the amount of PSS at the TiO2/water interface compared to that of PSS solutions with no CPBM added is indistinguishable within the experimental uncertainties. For these higher concentrations of CPBM, both PSS and CPBM appear to come to the TiO2 surface as aggregates and the mole ratio of CPBM to PSS at the TiO2/water interface decreases as the concentration of PSS is increased. For a CPBM concentration of 1.02 x 10(-5) M, the mole ratio of CPBM to PSS changes from 139 +/- 29 to 1 to 33 +/- 7 to 1 as the solution PSS concentration is increased. For a CPBM concentration of 1.04 x 10(-4) M, the mole ratio of CPBM to PSS changes from 630 +/- 130 to 1 to 110 +/- 21 to 1 as the solution PSS concentration is increased. Despite the large differences in the CPBM to PSS mole ratios, the amount of PSS that adsorbs to the surface is statistically indistinguishable for CPBM concentrations of 0, 1.02 x 10(-5), and 1.04 x 10(-4) M, indicating that the structure of the PSS molecules in each of the systems does not significantly change M the presence of CPBM.