The interfacial layer formed between a lithium-ion conducting solid electrolyte, Li7La3Zr2O12 (LLZ), and LiCoO2 during thin film deposition was characterized using a combination of microscopy and electrochemical measurement techniques. Cyclic voltammetry confirmed that lithium extraction occurs across the interface on the first cycle, although the nonsymmetrical redox peaks indicate poor electrochemical performance. Using analytical transmission electron microscopy, the reaction layer (∼50nm) was analyzed. Energy dispersive X-ray spectroscopy revealed that the concentrations of some of the elements (Co, La, and Zr) varied gradually across the layer. Nano-beam electron diffraction of this layer revealed that the layer contained neither LiCoO2 nor LLZ, but some spots corresponded to the crystal structure of La2CoO4. It was also demonstrated that reaction phases due to mutual diffusion are easily formed between LLZ and LiCoO2 at the interface. The reaction layer formed during high temperature processing is likely one of the major reasons for the poor lithium insertion/extraction at LLZ/LiCoO2 interfaces.