Phase equilibria and stability of a newly developed single phase equiatomic CoCuFeMnNi high entropy alloy (HEA) was investigated in the temperature range of 298-1573 K using in situ high temperature Xray diffraction and thermodynamic modeling using CALPHAD (CALculation of PHAse Diagrams) approach. Complimentary characterization techniques like scanning and transmission electron microscopy with energy dispersive spectroscopy for microstructural investigation and compositional analysis, differential scanning calorimetry for thermal analysis and atom probe tomography for near-atomic scale chemical analysis were employed. It was found that the metastable FCC solid solution alpha phase (lattice parameter = 0.361 nm) undergoes phase transformation at 923 K and 1123 K. The alpha phase transforms to BCC beta phase (a = 0.280 nm) at 923 K and on further heating, another FCC phase gamma (a = 0.362 nm) precipitates out at 1123 K, leading to coexistence of two FCC phases and one BCC phase. Atom Probe Tomography carried out to study the three-dimensional distribution of constituent elements indicates the presence of similar to 2 nm sized Cu clusters in the alpha phase. CALPHAD predictions indicate the tendency towards phase separation involving iron-cobalt and copper to partition out of the equiatomic solid solution to form the BCC and second FCC phase, respectively. A qualitative agreement between CALPHAD predictions and in situ high temperature X-ray diffraction accompanied with complimentary characterization tools explicitly demonstrates the fidelity of CALPHAD modeling for the design and development of novel HEA compositions. (C) 2017 Elsevier B.V. All rights reserved.