The average EEO2 value for the six graphene oxide NPs was ~1.9 x 10 -3 Da/O2 collision, but with large NP-to-NP variation. Four NPs with M near 35 MDa had EEO2 values just above 10 -3 Da/O2 collision, a ~25 MDa NP was six times more reactive, and as with graphite, the smallest NP (~9 MDa, Fig. ) was an order of magnitude less reactive than average (EEO2 ≈ 3.3 x 10 -4 Da/O2 collision). The graphene oxide feedstock is just a few layers thick, and for the mass range here, such NPs would have basal planes with lateral dimensions in the tens to few hundred nm range, i.e., their actual surface areas would be much larger than is assumed in calculating EEO2. The EEO2 values, therefore, significantly overestimate the true EEO2 values. Keep in mind however, that O2 reactivity at perfect basal plane sites is negligible, thus reactivity really depends not on the surface area, but rather the number of exposed reactive sites. For graphene oxide NPs the number should be large because the feedstock has a 30% O concentration, and CO loss as the NPs are heated should generate a high density of basal plane defects.