The effect of various benzenesulfonamide (BSA) plasticizers on the amorphous phase of semicrystalline polydodecamide (PA-12) has been investigated. MonoBSAs appear as efficient glass-transition temperature (T-g) depressors because of their miscibility with the host polyamide (PA), low glass transition, and small molecule size. PA-12's T-g shifts from 50 to about 0 degreesC at 20 mol % of the most efficient molecules. Comparatively, the more bulky bisBSAs appear to induce less important absolute T-g decreases (30 K at 20 mol %), although these appear as more important when considering the polymer T-g to plasticizer T-g difference. This unexpected observation could be ascribed to both the amide-sulfonamide interactions and the sterically generated disorder within the polyamide because of the plasticizer molecule's size. Phase-separation behavior of BSA plasticizers within the host PA has also been investigated. Crystalline phenyl-SO2NH2, for instance, dephased beyond 20 mol % in PA-12, forming distinct 1-2 micrometer wide crystalline domains as a result of its high propensity to crystallize upon cooling from the melt. By contrast, slow crystallizing N,N-dimethylBSA, which lacks any specific interaction for PA-12, remained nevertheless dispersed at a molecular level (metastable state, no phase separation) when vitrification of the host PA-12 amorphous phase occurred on cooling.