A polyester or polyesteramide capable of forming an anisotropic melt containing residues derived from dicarboxylic acids, dihydric phenols, hydroxyaromatic acids and aminophenols characterised in that at least 10 mole % of the acid residues of the polymer are radicals derived from fumaric acid or a substituted fumaric acid. Preferably at least 10 mole % of the residues other than the residues of fumaric acid or substituted fumaric acid are aromatic ring-substituted residues. The polyester or polyesteramides of the invention are particularly suitable for blending with chlorine-containing polymers at concentrations of up to 25% by weight of the polymer blend to improve the processability of the chlorine-containing polymer. A polyester in which the acid residues are entirely aliphatic is particularly suitable for blending with poly(vinyl chloride).