HF concentration and generation rate in organic LiPF₆-based carbonate electrolytes at 60°C was studied. Various compositions of linear and cyclic carbonates were used to determine the electrolyte degradation mechanism. Diethyl carbonate (DEC) has been identified as a promoter of HF formation, yielding 1 mmol/L of HF in 1 mol/L LiPF₆ DEC after 200 hours at 60°C. Prolonged aging of 1 mol/L LiPF₆, EC:DEC electrolyte at room temperature results in a significantly lower HF formation rate with time. Spiking the aged electrolyte with 500 ppm of water restores the HF formation rate to a level typical for a freshly prepared electrolyte.