The application of photocatalytic oxidation (PCO) in VOCs degradation is greatly hindered at high humidity levels. This is because VOCs compete with water molecules to adsorb on the generally hydrophilic photocatalyst surface, where photocatalytic reactions take place. Modified P25-TiO2 nanoparticles with surface fluorination (FP25) was prepared to reduce the surface hydrophilicity of Degussa P25. The prepared samples were characterized by BET, SEM, and XPS tests. Herein, the effects of surface fluorination on the adsorption capacity of P25-TiO2 nanoparticles towards toluene, methyl ethyl ketone (MEK), and isobutanol, representing different classes of indoor air pollutants, were investigated. After surface fluorination, the adsorption capacity of modified TiO2 was compared to bare-TiO2 in a continuous reactor at four different relative humidity levels (i.e., 0, 20, 40, and 60%). Three adsorption isotherms, including Langmuir, Freundlich, and BET, were used to model the adsorption experimental data. The equilibrium data for the adsorption of all compounds showed the best fit with the BET model, and the Freundlich model also represented a good fit. Moreover, the results indicated that the surface fluorination of P25 increased adsorption capacity about two times for toluene in three humid conditions (0, 20, and 40%) compared to bare-P25. By combining the benefits of using an easy modification method by a low-cost modifier and using P25-TiO2, which is the most common commercialized photocatalyst, an effective method has been developed to enhance the efficiency of VOCs removal in indoor air environments.