As for Li-and Na-ion batteries, the electrolyte reactivity (i.e. the salts and solvents decomposition) also plays a crucial role on the electrochemical performance of K-ion batteries (KIBs). However, X-ray photoelectron spectroscopy (XPS) analysis of the solid electrolyte interphase (SEI) in KIBs remains based on Li-ion and Na-ion literature so far. This may lead to incorrect interpretations of the results considering that the first ionisation potential difference between the alkali metals is expected to significantly change the binding energy values observed by XPS for a given M-based (M=Li, Na or K) compound. Therefore, this work aims at providing XPS characterization of numerous commercial K-based reference compounds, including some that were still missing in the literature. In particular, absolute and relative binding energy values are given and compared to corresponding Li and Na counterparts. This work will thus help researchers to get reliable XPS analysis of the SEI in KIBs. KEYWORDS K-ion batteries, solid electrolyte interphase, XPS, K-based reference compounds, binding energy values.