We report the direct observation of solid phase electrolyte products resulting from decomposition of the acetonitrile solvent on the mesoporous activated carbon positive electrode of a nonaqueous asymmetric hybrid supercapacitor by C-13, H-1, and N-15 solid-state nuclear magnetic resonance (NMR). At least two of the observed NMR signals can be assigned to acetamide and a lithium-based carbon derivative, which suggests that the Li-salt interacts with the acetonitrile solvent, leading to accumulation of the decomposition products on the electrode particle surface. These results may find practical applications in evaluating the design and performance of high-voltage electrochemical double layer capacitors and lithium-ion batteries. (c) 2006 The Electrochemical Society.