For bulk Si surfaces, passivation requires Oads coverages high enough to allow nucleation and growth of SiO2, and thus is favored by low temperatures (longer Oads lifetimes) and/or high PO2 (faster Oads formation). Conversely, at high T and/or low PO2, desorption of species like SiO, O, and O2 is fast, keeping Oads coverage low and preventing passivation. In our study of O2 etching of Si NPs, most experiments were done at PO2 = 4.3 x 10 -5 Torr (same as most experiments here) and TNP ranging from 1200 to 1750 K. The entire set of reaction conditions studied was in the range where bulk Si surfaces are not observed to passivate. Nonetheless, for TNP ≤ 1500 K, Si NPs were observed to passivate if the experiments continued long enough, losing >50% of Minitial in the process.