Nanoparticles (NPs) are increasingly being incorporated into consumer products and are being used for industrial applications in ways that will lead to their environmental dissemination via wastewater treatment plants (WWTPs). Many NPs possess catalytic properties that could potentially enhance undesired chemical reactions such as the formation of disinfection by-products during disinfection of wastewater effluent. In this effort, silver (AgNPs), titanium dioxide (TiO2), ceria (CeO2), and nano zero valent iron (NZVI) NPs were investigated for their potential to enhance trihalomethane (THM) formation in three different disinfection regimes: UV alone, free chlorine, and UV+free chlorine. Of the test nanomaterials, only AgNPs demonstrated the capacity to enhance THM formation and thus they were subjected to additional study. AgNPs enhanced THM formation at all concentrations examined (1, 10, and 20 mg L-1) even though the AgNPs were chemically unstable in the presence of free chlorine. The transformation of the AgNPs and the production of non-metallic silver species was observed via UV-vis spectroscopy. The capacity for AgNPs to enhance THM formation was considerably increased in the UV+free chlorine disinfection regime. Although not the focus of the study, formation of AgNPs during UV disinfection of Ag+ in the effluent was also observed. This study illustrates the potential for NPs to catalyze unfavorable chemical reactions during WWTP effluent disinfection. Such a result could prove detrimental to aquatic receiving environments and is especially of concern in water reuse scenarios where aggressive disinfection regimes may be utilized.