The composition and synthesis conditions of the (Li/Na)CoO(2) phase with an ordered 1:1 Li/Na stacking alternating with CoO(2) slabs were determined from a careful study of the P2-Na(∼0.7)CoO(2)-O3-LiCoO(2) system. An in situ X-ray diffraction (XRD) thermal study emphasizes the metastable character of this phase that can be stabilized only by very fast quenching. Its composition, (Li(0.42)Na(0.37))CoO(2), is significantly different from the ideally expected one, (Li(0.50)Na(0.35))CoO(2), and its structure, confirmed by Rietveld refinement of the XRD pattern, presents an ideal alternate ordering of lithium, cobalt, and sodium layers within OP4-type oxygen packing. The presence of vacancies in both alkali-ion layers was confirmed by electrochemical intercalation of lithium and sodium. For the first time, a new type of layered oxide exhibiting OPP9-type oxygen packing was evidenced. Between the CoO(2) slabs, alkali ions are intercalated in the following order: Li(octa)-Na(prism)-Na(prism). This material crystallizes in the R3m space group with a(hex) = 2.828 Å and c(hex) = 46.85 Å cell parameters.