The phase behaviour of the ternary blend system polyamide 6/polyamide 66/elastomer has been interpreted by the binary interactions of the respective polymers. Three different elastomer components have been studied; random ethene-propene copolymer (EPM), ethene-propene-diene copolymer grafted with maleic anhydride (EPDM-g-MA) and ethene-1-octene copolymer also grafted with maleic anhydride (EO-g-MA), respectively. The miscibility of the polymers is predicted by the Flory-Orwoll-Vrij equation-of-state theory using the pVT data in connection with the solubility parameter concept. The data suggests that polyamide 6 (PA 6) and polyamide 66 (PA 66) are miscible within the practically accessible temperature range from 270 to 310 degrees C and all types of elastomer are immiscible with both polyamides due to strong repulsive interactions mainly characterised by large differences of the characteristic pressure between polyamides and elastomers. Thus the interaction parameters chi(ij) between the polyamides and the different elastomers exceed the critical chi parameter unambiguously. Therefore, all ternary systems are heterogeneous. This is confirmed by dynamic mechanical analysis experiments where a single glass transition was only observed for PA 6/PA 66 blends, whereas two glass transition temperatures are observed for all ternary blends. The heterogeneous systems show a glass transition temperature depression for the ethene-propene copolymer and ethene-octene copolymer grafted with maleic anhydride depending on the composition in the ternary blends.