We have previously reported use of single NP mass spectrometry to measure kinetics for high temperature reactions including sublimation, etching, and growth of surface layers, for NPs of different types of carbon materials, silicon, silica, and semiconductor quantum dots. Here, we report the first measurements of sublimation and reactions with O 2 for hafnium (Hf) NPs, extending the maximum T NP range to match the more refractory nature of Hf. The Hf results are compared to analogous experiments for graphite, carbon black, and silicon NPs to obtain insight into the mechanisms for Hf etching and passivation. We also discuss the factors that limit the method and the sources of uncertainty in both the absolute mass measurements and the kinetics that result from measurements of mass vs. time. The approach to measuring T NP by fitting single NP emission spectra is outlined, and the first examples of the emission spectra from individual Hf NPs are presented.