The melt of polydodecamide (PA-12) shows a significant viscosity decrease upon incorporation of benzenesulfonamide plasticizers (BSAs), this effect being maximum for a monofunctional BSA with a 12-carbon-atom-long alkyl chain. Nonexhaustive X-ray diffraction analysis developed on isothermally crystallized samples validated a two-phase model for describing PA-12 plasticized by N-(n-butyl)benzenesulfonamide (BBSA). The massive presence of BBSA between the lamellar crystals was established, and lamellar fragmentation was also observed. Further, a steady increase in PA-12 crystallinity with an increasing BBSA content was evident (and confirmed by DSC) and is consistent with the plasticizer easing the mobility of polymer chains during crystallization. Large melting point depressions resulting from both polymer-plasticizer miscibility and lamellar fragmentation were observed with several mono- and bifunctional BSA plasticizers. Phase separation in PA-12 solid state was only observed at 20 mol % of phi SO2NH2, alhough miscibility occurred in the melt.