The structure-properties relationships of a polyamide 12 (PA12)/polyamide 6 (PA6) blend containing multiwalled carbon nanotubes have been investigated as a function of nanofiller amount. The influence of the filler content in the water diffusion behavior of the immiscible PA12/PA6 blend is studied. An acceptable dispersion level of nanotubes in polymer matrix is observed by scanning electron microscopy. The water uptake at saturation, normalized to the polyamide mass, decreases, and the diffusion rate is slower in CNT composites. The enhancement is in line with the increase in stiffness measured in rheological tests. This behavior is related with the rise in the overall crystallinity of the polymer matrix and the selective migration of CNT towards PA6 (more hygroscopic than PA12). These findings display a substantial improvement in respect to the water uptake behavior of pristine PA6 and PA6/CNT composites, expanding the potential applications of composites.