Dimethyl sulfoxide (DMSO) is electrochemically co-intercalated with lithium ions into graphite to form a stable ternary graphite-intercalation-compound (GIC), Li-DMSO-GIC. To elucidate the details of the co-intercalation behavior, in situ Raman spectroscopy was performed to observe the structural variations in graphite during the potential scan. During the scan from 2.44 to 1.00 V, DMSO was co-intercalated with lithium ions into graphite, exhibiting a staged structure. The stage number decreased from 4 to 1. During the scan from 1.00 to 2.44 V, the deintercalation of DMSO-solvated lithium ions was observed. After experienced a potential cycle between 2.4 and 0 V, the graphite exhibited significant changes that are ascribed to the exfoliation of graphite, implying that the destruction of the graphite composite electrode is likely to be caused by the decomposition of DMSO within the graphite layer.