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Many transition metal sulfides and phosphides are susceptible to surface oxidation under ambient conditions.The formed surface oxidation layer,which is likely to further restructure under reaction conditions,alters the chemical properties of the pristine material but has not been well studied.In this work,we for the first time use X-ray photoelectron spectroscopy to quantify the natural surface oxidation of transition metal phosphide and sulfide nanoparticles and employ a simplified Deal-Grove model to analyze the kinetics.We show that CoS2 oxidizes faster than CoS whereas CoP2 is more difficult to oxidize compared to CoP,and there exists an inverse correlation between the surface oxidation rate and the Co-S/P distance in the pristine structure.More inclusive investigation unveils different types of surface oxidation behavior:CoS,NiS and FeS are limited by their reactivity with oxygen;CoS2 is the most reactive and its oxidation is governed by oxygen diffusion;CoP2 is influenced by both reactivity and diffusion;CoP,Ni2P,Cu3P and MoP exhibit high initial oxidation degrees and the kinetics are not well-defined;MoS2 is largely stable against oxidation.