We synthesized the cobalt sillenite with overall stoichiometry Bi24Co2O39-delta using conventional ceramic route and characterized it in detail. XRD and EDS were used to determine phase purity and Rietveld refinement was applied to determine lattice parameters. Relaxation time calorimetry and DSC were used to measure low temperature heat capacity and to evaluate the molar entropy. Low temperature heat capacity was then analyzed by a combination of Einstein and Debye model. Thermal stability and oxygen non-stoichiometry were analyzed using STA. Drop experiments were performed to evaluate the dependence of high-temperature heat capacity. Magnetic properties were studied by vibrating sample magnetometry. The obtained results were applied to re-assess the thermodynamic model of sillenite phase and the ternary Bi-Co-O phase diagram. This knowledge is essential for the construction of the entire Bi-Sr-Co-O phase diagram involving ternary phases with outstanding thermoelectric properties. (C) 2015 Elsevier B.V. All rights reserved.