Several points are immediately obvious. In no case was measurable mass gain observed, even when the NPs were initially exposed to O 2 . Instead, the NPs lost mass, with rates that fluctuated to varying degrees, and then ultimately passivated, i.e., the mass loss rates decreased substantially. As shown more clearly in Fig. , however, the percent of the initial mass lost before the NPs passivated varied substantially, with carbon black passivating after ~7% mass loss and graphite after ~37% mass loss, with case. The dynamics of the active-etching-to-passivated transitions for the four NPs were, however, quite different, as shown by the dependences on total mass loss (Fig. ) and reaction time (Fig. ).