The influence of a filler on melt-mixed conducting polymer blends of poly(3-octylthiophene) (POT) has been examined with respect to their morphology, rheology, conductivity and acid-base properties. The matrix polymers used were low density polyethylene (LDPE), poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) and the filler was non-conducting aluminum borate whiskers. These blends show two-phase behavior when examined by scanning electron microscopy. It was found that the adhesion at the polymer-filler interface in combination with the viscosity ratios between the polymers exerts a considerable influence on the morphology and hence on the conductivity. For the PE/POT blends, addition of whiskers changed the morphology and increased the conductivity by several orders of magnitude. The conductivity of blends with PVC was almost unchanged while the conductivity of PMMA blends slightly decreased upon addition of whiskers. The interactions between whiskers and the polymers used decreased in the order: PMMA > POT > PVC > PE. This is in accordance with the Lewis acid-base properties determined by inverse gas chromatography.