Exploration of the adsorption mechanism for mixtures at photocatalyst surfaces is a prerequisite for a full understanding of photocatalytic oxidation (PCO) technology for treatment of gaseous contaminants in indoor air applications. However, there has been very little work on the competitive adsorption of photocatalysts. In this article, an experimental and analytical study on the co-adsorption of nine binary mixtures on a commercial PCO filter was investigated using a bench-scale single-pass continuous flow system. Adsorption tests were performed with a concentration of 500 ppb for the selected mixtures at various molar ratios. The experiments were performed at 40% RH, 21 degrees C, and a gas flow rate of 10L/min. Quantitative methods were developed to describe inhibitory and facilitatory effects on the adsorption of one component by the other. It was found that for the non-polar mixture of p-xylene and toluene, the polar mixture of MEK and acetone, and the polar/non-polar mixture of MEK and p-xylene, adsorption selectivity varied from 0.83-1.81, 1.80-1.21, and 2.60-1.78, respectively, when mixing ratio of each mixture changed from 1:2.33 to 2.33:1. In addition, a time-dependent co-adsorption model was developed and validated with the experimental results. It was concluded that competitive adsorption performance is dependent upon the composition of the gas mixture, the natures of the adsorbate and substrate, and the initial molar ratio of VOCs.