Miscibility of a series of nylons-6,m with poly(hexamethylene isophthalamide), abbreviated 6I nylon, was investigated. Four polyamides (nylon-6,5, nylon-6,7, nylon-6,8, and nylon-6,9) were found miscible, on a DSC scale, with 6I nylon. A series of nylon-n,m which had the same number of CH2 groups as nylon-6,7 was also studied; no miscible blend with 6I nylon was found. This was taken as direct evidence of the importance of average interamide distances for miscibility. Negative deviations from linearity in T-g-composition behavior of the blend are proposed to stem from conformational changes leading to weaker van der Waals interactions. These weak interactions are counterbalanced, In terms of energy, either by more ordered hydrogen bond networks or by entropy effects. For nylon-6,8/6I nylon blend, an S-shaped curve was observed, which is attributed to the occurrence of variations of Chain morphologies with blend composition.