The Cu–Sn and Cu–Nb–Sn systems were optimized by the means of the CALPHAD (CALculation of PHAse Diagram) technique. The solution phases (liquid, fcc, bcc and bct), gamma and Cu 10 Sn 3 were described by the substitutional-solution model. The compounds Cu 3 Sn and Cu 41 Sn 11 in the Cu–Sn system were treated as (Cu,Sn) m (Cu,Sn) n . The compounds η-Cu 6 Sn 5 and η′-Cu 6 Sn 5 in the Cu–Sn system were treated as Cu(Cu,Sn)Sn by a three-sublattice model. The compounds Cu 3 Sn, Cu 41 Sn 11 , η-Cu 6 Sn 5 and η′-Cu 6 Sn 5 in the Cu–Sn system have no solubilities of the third component Nb in the Cu–Nb–Sn system. The compounds Nb 3 Sn, Nb 6 Sn 5 and NbSn 2 were treated as (Nb,Sn) 3 (Nb,Sn), Nb 24 Sn 16 (Nb,Sn) 4 and (Nb,Sn)(Nb,Sn) 2 in the Nb–Sn system, respectively. The compounds Nb 3 Sn, Nb 6 Sn 5 and NbSn 2 in the Nb–Sn system were treated as (Cu,Nb,Sn) 3 (Nb,Sn), (Cu,Nb) 24 Sn 16 (Nb,Sn) 4 and (Cu,Nb,Sn)(Nb,Sn) 2 in the Cu–Nb–Sn system, respectively. A self-consistent thermodynamic description of the Cu–Nb–Sn system was obtained.