Miscibility in blends of semi-aromatic polyamides and random aliphatic co-polyesteramides, derived from epsilon-caprolactam, epsilon-caprolactone and laurolactam has been investigated by calorimetric methods. Dilution of epsilon-caprolactam by epsilon-caprolactone in PA-6 transforms miscible blends based on PA-6I and PA-TMDT, respectively, to immiscible mixtures at approximately 35 and 46 mol%, respectively, of epsilon-caprolactone in the copolymer. The data have been found to conform to an analysis provided by a binary interaction model incorporating segmental interaction parameters, chi(ij), derived from related blends. Extension of the treatment to polyamide/polyester blends whose constituents contain aromaticity, e.g. PA-66/poly(ethylene terephthalate), PET and PA-6I/polyarylate, PAr, has also been explored. The results confirm the gross immiscibility, indicated by a large unfavourable interaction parameter, typically encountered in these blends.