The influence of the electrolyte in lithium-ion batteries on the reversible capacity and long-term cycling stability is not only determined by the salt and the film-forming solvent, but also, to a minor extent, by the co-solvent that is usually comprised in order to ensure suitable physical properties. Electrolytes containing three different isomers of butyl methyl carbonates were investigated in both, graphite half- and full-cells in order to determine the influence of co-solvent chain branching on the electrochemical performance. The reversible capacity obtained in SFG44 graphite (TIMCAL Ltd.) half-cells is higher for electrolytes with branched co-solvents and increases as the chain branching gets closer to the carbonate group. The long-term cycling stability, investigated in SLP30 graphite (TIMCAL Ltd.) half-cells, shows the same trend. The overall effect is rather small, but still perceivable in a full-cell set-up (SFG44/LiCoO2). A possible pathway of interaction of the co-solvent via the structure and composition of the solid electrolyte interphase (SEI) is proposed.