Understanding the toxicity of carbon nanomaterials, such as carbon nanotubes and graphenes, is important for the development of nanotechnology. Studies have shown that surface redox capability is an important factor for toxicity of carbon nanomaterials. We have measured the surface reductive capacity for a number of carbon nanomaterials in previous studies, but the effects of various engineering processes on surface redox capability have not been investigated until this study. In this study, commercially available carbon black, carbon nanotubes, standard reference materials, fullerenes, graphenes and acetylene soot generated in the lab were used. The carbon nanomaterials were subjected to heating at various temperatures in various atmospheres up to 500 ˚C, and soaking in water at room temperature under various atmospheres, and weathering in the powder form at room temperature under various atmospheres. The redox capability of the carbon nanomaterials was quantified in terms of the reductive capacity towards Fe3+ ions (RCFI). The RCFI values of the asreceived nanomaterials and that of the nanomaterials after various treatments were compared. The carbon nanomaterials were also characterized using x-ray photoelectron spectroscopy (XPS), for understanding the surface chemistry mechanisms of RCFI and the effects of various treatments. In general, heating induced a significant increase in RCFI, regardless of the atmosphere under which the nanomaterials were heated. On the other hand, aging in O2- containing atmospheres brought about significant decrease in RCFI, either in water suspension or in the powder form. Water vapor enhanced the aging effect of O2. CO2 was found to affect the RCFI and the aging of carbon nanomaterials. The extent of RCFI change due to heating or aging was dependent on the type of material. According to the XPS results, the RCFI of some carbon nanomaterials such as carbon black may be correlated with the C-O surface functional groups. However, the definitive correlation between the oxygen-containing surface functional group and RCFI for all carbon nanomaterials couldn’t be determined by the XPS result. This indicates that the RCFI changes of carbon nanomaterials after treatments mainly derived from the factors such as the active sites of edges other than the oxygen-containing surface functional group changes as other studies show. This suggests that the RCFI measurement cannot be replaced by XPS analysis. The effects of heating and aging on RCFI, and more generally the surface redox capability of carbon nanomaterials, reveals that various engineering and environmental processes may significantly change the toxicity of carbon nanomaterials. The findings of this study suggest that it is important to take into account the effects of engineering and environmental processes when assessing the toxicity of carbon nanomaterials.