Solution-based combinatorial samples of lithium cobalt manganese oxide were synthesized and studied by X-ray diffraction in order to map out the pseudoternary phase diagram over the entire metal composition ranges. This report focuses on the single-phase layered region found to be made up of a single composition line joining LiCoO2 to Li2MnO3. The solid solution was found to phase separate during slow cooling (1 °C/min). The end points of this coexistence are not LiCoO2 and Li2MnO3: instead the Co-rich phase contains some Mn and the Mn-rich phase contains some Co. This phase separation occurs at compositions where previous studies showed nanodomain phase separation when an intermediate cooling rate was used. The nanoscale composites are therefore an intermediate structure that forms when there is insufficient time during cooling for large scale crystallites of the new phases to form. A simple Monte Carlo simulation was used to illustrate this phase separation and study the impact of cooling rate.