With the increasing time people spend indoors, the indoor environment quality draws more and more attention. The concentration of indoor pollutants is usually much higher than outdoors, in which volatile organic compounds (VOCs) and mold/mildew are both major pollutants and cause many health problems to residents. Efforts devoted from academy and industry to protecting people from indoor environment problems are apparently not sufficient. Photocatalysts, such as TiO2, WO and ZnO, can absorb light photons and react with O2 and H2O to generate highly oxidative radicals, which can oxidize VOCs and disinfect microorganisms. Recently, this photocatalytic oxidation (PCO) technology has been intensively studied to reduce VOCs and disinfect bacteria in the indoor environment. Few papers address the indoor mold/mildew problem, and this research therefore endeavors to do so. The objectives are to evaluate the effectiveness of PCO technology to resist mold/mildew growth and prevent VOC emission from building materials under either UV or visible light irradiation. The models, including linear regression, logistic regression, and numerical model, are also built for interpreting experimental results and for predicting performance in application. The mold/mildew resistance of different PCO gels was examined using accelerated mold/mildew growth agar plate tests. These gels included TiO2 only and TiO2 in combination with H2O2 and with Ag. Without the application of PCO gels, no mold/mildew inhibition was observed from UV (365 nm) or visible light. Under UV light irradiation, the TiO2 gel achieved complete mold/mildew inhibition. Without light, a 12-day delay of mold growth was obtained using the Ag-TiO2/H2O2 gel. Under visible light irradiation, the Ag-TiO2/H2O2 gel was also the most effective PCO gel with a 8-day delay of mold growth, which, however, was shorter than the same gel in the condition of no light with a 10-day delay due to the light-induced deterioration of the Ag-TiO2. The reduction of VOC emission from PCO gel (TiO2 gel and Ag-TiO2/H2O2 gel) coated building materials under UV or visible light irradiation was also confirmed by small chamber tests (the Ag-TiO2/H2O2 gel with above 50% reduction of total VOC emission). A linear model was obtained for the Ag-TiO2/H2O2 gel in the condition of no light, with respect to the correlation between the delay of mold growth and the gel ingredients. A logistic model was created for predicting the probability of mold growth on different TiO2 gels with different UV light exposure time at different intensities. A numerical model was developed with better accuracy than the previous one for VOC emission from PCO gel coated building materials. This study showed that the PCO gel might be a promising multifunctional material in resisting mold/mildew growth and preventing VOC emission in the indoor environment (The TiO2 gel for complete mold/mildew inhibition and the Ag-TiO2/H2O2 gel for delay of mold growth in emergency situations and reduction of VOC emission from building materials). More stable Ag-TiO2 or other visible-light-driven photocatalysts are needed in future research because of the deterioration of the current one.