The shifting of lambda(max) (pi-pi* transition) of P3OT (poly(3-n-octylthiophene)) was used to monitor the degree of miscibility between P3OT and the matrix polymers. In the ternary blend system, 1% of P3OT is soluble in a uniform PMMA/EVA20 mixed matrix and does not interfere with the miscibility between PMMA and EVA20 (ethylene-vinyl acetate copolymer with 20% Of vinyl acetate). But 5% of P3OT can attract EVA20 out of the matrix and induce the separation from 95% of PMMA/EVA20 matrix and cause the doublet peak of lambda(max). Likewise, the DSC thermograms do not demonstrate the melting-point depression of EVA20 in a 5% P3OT ternary system. However, the melting-point depression did occur only for 1% of P3OT. IR-spectra demonstrate that the methylene groups affinity contributed to the misciblity either in 1% P3OT/PMMA or in P3OT (I and 5%)/EVA20. The solubility difference of P3OT in PMMA (poor) and EVA20 (better) creates a continuous morphology for obtaining a highly conjugated path in a nonconjugated matrix. (c) 2007 Wiley Periodicals, Inc.