We report the use of solvent vapor swelling of ultrathin polymer films to determine Flory-Huggins solvent-polymer and polymer-polymer interaction parameters (chi(i-j)) for poly(3-hexylthiophene) (P3HT) and polystyrene (PS) over a wide solvent composition range. From the calculated interaction parameters, we constructed a polymer/polymer/solvent phase diagram that was validated experimentally. chi(tetrahydrofuran-P3HT) (1.04 +/- 0.04) and chi(CHCl3)-(P3HT) (0.99 +/- 0.01) were determined through swelling of ultrathin P3HT films. Similar experiments using PS films gave chi(tetrahydrofuran-PS) = 0.41 +/- 0.02 and chi(CHCl3-PS) = 0.39 +/- 0.01, consistent with literature values. As expected, these chi(i-j) parameters indicated that P3HT is less compatible than PS with either From solvent delta(PS) From bps (17.9 +/- 0.2 MPa1/2) and delta(P3HT) (14.8 +/- 0.2 MPa(1/)2) determined through regular solution theory, we calculated chi(PS-p3HT) = 0.48 +/- 0.06 at 23 degrees C. The resulting phase diagram was validated by solution-based transmission measurements of PS/P3HT blends in o-xylene. Although we focused on PS/P3HT blends in this work, this approach is easily adaptable to other polymer/polymer combinations of interest.