To develop K-ion batteries, the potassium metal reactivity in a half-cells must be understood. Here, it is shown first that the K metal leads to the migration of the electrode degradation species to the working electrode surface so that half-cells' solid electrolyte interphase (SEI) studies cannot be trusted. Then, the K metal reactivity was studied by combining gas chromatography (GC)-mass spectrometry, GC/Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis after storage in ethylene carbonate/diethylene carbonate (EC/DEC) wo/w 0.8 M KPF6 or KFSI. A comparison with Li stored in EC/DEC wo/w 0.8 M LiPF6 was also performed. Overall, full electrolyte degradation pathways were obtained. The results showed a similar alkali reactivity when stored in EC/DEC with the formation of a CH3CH2OCO2M-rich SEI. For a MPF6-based electrolyte, the reactivity was driven by the PF6- anion (i) forming mostly LiF (Li metal) or (ii) catalyzing the solvent degradation into (CH2CH2OCOOK)2 and CH3CH2OCOOK as main SEI products with additional C2H6 release (K metal). This highlights the higher reactivity of the K system. With KFSI, the reactivity was driven by the FSI- anion degradation, leading to an inorganic-rich SEI. These results thus explain the better electrochemical performance often reported in half-cells with KFSI compared to that with KPF6. Finally, the understanding of these chemically driven electrolyte degradation mechanisms should help researchers to design robust carbonate-based electrolyte formulations for KIBs