The plasticizing effect of benzenesulfonamides (BSAs) on an amorphous aliphatic polyamide (AAPA) has been studied using dynamic mechanical analysis of copper-supported spin-coated mixtures. It follows that N-(n-butyl)BSA (BBSA), an amorphous liquid hydrogen bonding BSA,. is fully miscible with AAPA because their mixtures are characterized by a single glass transition (T-g) throughout the compositional range. The T-g-composition dependence, however, is not linear because experimental results suggest a 20 K fall in T-g occurring around 0.65 BBSA units per amide unit, which coincides with the system shifting from a polymer-like to a liquid-like glass-forming material. When considering a crystallizable hydrogen-bonding plasticizer such as ethylBSA (EBSA), AAPA/EBSA mixtures become fully crystalline at a 1.3 EBSA unit per amide group. Nevertheless, melting point depression together with the single T-g observed throughout the compositional range on quenched (and therefore amorphous) samples confirms the miscibility of AAPA chains with the plasticizer. N,N-DialkylBSAs, which lack the sulfonamide proton and therefore the possibility of hydrogen bonding with amide groups, quickly phase separate from AAPA, the glass transition of the latter staying mainly unaffected apart from a small (9K) decrease at 10-15mol% plasticizer. (C) 2001 Society of Chemical Industry.