Engineered nanomaterials are beginning to make their way into a variety of consumer goods and biomedical applications.  In parallel to the development of nanomaterials into these products, the need for regulatory guidelines are critical to success of these novel technologies.  One important factor within the regulatory discussion is the accurate and relevant nanomaterial characterization methods of the physicochemical properties for toxicological evaluations.  Some of the most common properties of nanomaterials in aqueous suspensions are aggregation/agglomeration, leaching of metal ions, and the production of reactive species.  With some nanomaterials, when designed appropriately, they can generate reactive oxygen species quite efficiently.  In our studies, we characterize the toxicity of several classes of nanomaterials and find that under ambient conditions in cell culture and in the animal model, these substances are cytotoxic and inflammatory at different concentrations over different time points.  An examination of the differential toxicities of different surface area samples found little relation between surface area and toxicity.  In contrast, the phase composition and surface coating of the nanomatertial was highly related to cytotoxicity.  We also how that the observed toxicity of our materials relates strongly with their ability to generate ROS species, ex vivo.   These data suggest that users of certain nanomaterials may rely on ex vivo measures of physicochemical properties as indicators for safety assessment.