We have shown the ability of different types of oxide nanoparticles, CeO2, Fe2O3, TiO2 and SnO2 to skew the accuracy of detection of H2O2 using the widely applied Amplex Red assay. These oxide materials are commonly employed in electronics manufacture, industrial catalysis, and in the manufacture of consumer products, and their toxicological profiles are not well understood. All four oxide nanoparticles demonstrated intrinsic capability of converting Amplex Red molecules to fluorescent resorufin, even in the absence of H2O2, in aqueous solution as well as cell culture media. Kinetic parameters were calculated and a simulated time dependent concentration profile was generated to better understand this complicated system. The results show that care is needed for interpreting the result of seemly robust bioassays in the presence of nanoparticles.