The phase diagram of the binary system poly(3-octylthiophene) + poly(ethylene-co-vinylacetate), P3OT + EVA was studied by Differential Scanning Calorimetry (DSC). The blends of low and high content of P3OT were found to be homogeneous. The system was also studied by a dielectric relaxation technique in the frequency range 20 Hz-100 kHz. The complex electric modulus M* is discussed in terms of the well-known Havriliak-Negami function. We also obtained the spectrum of relaxation times using a regularization technique. The spectra allowed us to calculate the d.c. conductivity. A low frequency contribution to the dielectric loss found in two-phase samples was attributed to interface polarisation effects.