We report a robust methodology based on linear sweep voltammetry to determine experimentally the electrochemical oxidative stability of hydroborate-based solid-state electrolytes for all-solid-state batteries. To accelerate kinetics and improve the sensitivity to decomposition, we explore different solid-state electrolyte/carbon composites and employ a low scan rate of 10 μV s–1. Using LiBH4 as a model system, we show that proper selection of the conductive carbon and its ratio in the composite are important for an accurate determination of the intrinsic oxidative stability. This method is robust with respect to the choice of the current collector material and the ionic conductivity of the solid-state electrolyte. The measured oxidative stability of LiBH4 of 2.0 V vs Li+/Li is in good agreement with the value predicted by first-principles calculations. The irreversible electrochemical decomposition of LiBH4 outside the oxidative stability limit is independently confirmed by galvanostatic cycling. We apply this method to reassess the electrochemical oxidative stability of selected, highly conductive hydroborate-based solid-state electrolytes, including Li2(CB9H10)(CB11H12), Na3(BH4)(B12H12), Na4(B12H12)(B10H10), and Na4(CB11H12)2(B12H12), and emphasize the necessity of selecting cathode materials for all-solid-state batteries based on the accurate understanding of the oxidative stability of the solid-state electrolytes.