Polybutylene terephthalate (PBT) and highy density polyethylene (HDPE) were blended and stabilized with four different stabilizers obtained from the organic derivatives of five-valent phosphorus. These stabilizers were cyclohexylphosphonic acid, its metallic salts of cyclohexyl phosponates of bivalent nickel, bivalent iron and trivalent iron. The stabilization improved the thermal stability of the PBT/HDPE blends and increased their mass loss temperature. The blend melts were thermally aged inside a viscometer barrel and the Melt flow Index (MFIs) was measured. All stabilizers increased the melt stability of the blends when used in low rates (< 0.2% in mass). At higher stabilizer rates, some stabilizers further enhanced the thermal stability, while others decreased it. A melt parameter was introduced to determine the melt stability of the PBT/HDPE blends during extrusion. The best stabilization was obtained with cyclohexylphosphonic acid based on the metallic salts of bivalent nickel. The mechanisms that lead to the inhibition of thermal destruction of the blends were explained in terms of the stabilizers.