© 2017 American Physical Society. We present a calculation of the η-η′ mixing in the framework of large-Nc chiral perturbation theory. A general expression for the η-η′ mixing at next-to-next-to-leading order (NNLO) is derived, including higher-derivative terms up to fourth order in the 4-momentum, kinetic terms, and mass terms. In addition, the axial-vector decay constants of the η-η′ system are determined at NNLO. The numerical analysis of the results is performed successively at leading order, next-to-leading order, and NNLO. We investigate the influence of one-loop corrections, Okubo-Zweig-Iizuka rule-violating parameters, and O(Ncp6) contact terms.