Inorganic oxide nanoparticles (NPs) are used in semiconductor manufacturing operations such as wafer chemical–mechanical planarization (CMP). Understanding the stability of NPs in municipal wastewater is essential for the evaluation of the fate of NPs released to municipal wastewater treatment plants (WWTPs). This study aimed to evaluate the stability of Al2O3, CeO2, and SiO2 NPs and CMP waste effluents containing these NPs in municipal wastewater. Al2O3 and CeO2 NPs were destabilized by wastewater constituents, as indicated by the formation of large agglomerates. However, the same NPs in the CMP waste slurries showed high stability in wastewater, probably due to additives present in the slurry that modify the surface chemistry of the particles. Likewise, both the commercial SiO2 NPs and the CMP waste slurry containing SiO2 NPs showed substantial stability in wastewater since this NP has a very low point of zero charge, which suggests that this NP could be the hardest one to remove in conventional WWTPs by aggregation–sedimentation. In summary, the results indicate that wastewater may destabilize NPs suspensions, which would facilitate NP removal in WWTPs. However, some chemicals present in real CMP slurries may counterbalance this effect. More research is needed to completely understand the surface chemistry involved.