eta-eta' mixing from U_L(3) x U_R(3) Chiral Perturbation Theory

We obtain explicit expressions for the \eta and \eta’ masses and decay constants using U_L(3) x U_R(3) chiral perturbation theory at next-to-leading order in a combined expansion in p^2 and 1/Nc. A numerical fit of the parameters appearing to this order of the expansion is also discussed.