We analyze interfaces between A-righ and B-rich phases in symmetric ternary polymer blends with a non-selective C minority component. The interfacial tension σ and the interfacial thicknesses of the composition profiles of φA(x) − φB(x), and φC(x), L1 and L2, respectively, are calculated by solving the linearized equation resulting from minimizing σ as a function of the average composition of C, bar varphiC = 1 − bar varphi, with bar varphi/2 = bar varphiA = bar varphiB, and the net interaction per thermal energy between components A and B, χAB = χ (χAC = χBC = ηχ). As bar varphiC increases the interfacial thickness increases and the interfacial tension decreases. Our results are in excellent agreement with the steady-state solution of the non-linear spinodal decomposition equations.