Superheated water (shH(2)O) is investigated as a process aid in conventional aliphatic polyamide (PA) systems. The polymers investigated include PA 6 (PA6), PA 6,6 (PA66), PA 6,12 (PA612), and PA 12 (PA12). It is shown that the PA melting and crystallization temperatures are significantly reduced when exposed to shH(2)O. For example, the melting temperature of PA6 is depressed from 206 to 153 degrees C in the presence of shH(2)O. A relationship between amide group density and thermal transition temperature reduction is observed. Processing these materials in shH(2)O has led to a variety of materials ranging from low-density foams to higher density locally anisotropic foamed morphologies. In situ observations of PAs melting in the presence of shH(2)O are performed using a specially designed reactor. Results from these experiments are used to estimate the diffusion coefficient of shH(2)O in PA6. Finally, low-temperature extrusion is performed with PA6 and shH(2)O at temperatures as low as 180 degrees C and mixture viscosity is estimated. A 20-fold depression in the melt viscosity of PA6 is observed at 240 degrees C with shH(2)O. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 803-813