Poly(6-hexanelactam)-block-poly(12-dodecanelactam) copolymers containing a minor part of poly(6-hexanelactam) (PA6) and poly(1 2-dodecanelactam) (PA12) were compared with PA6-PA12 blends as regards their crystallization behaviour, thermal properties, sorption of water vapour, morphology and mechanical properties. All blends were prepared by the combined polymerization procedures described earlier. The differences found are ascribed to the different composition of amorphous phases, specifically to the compatibilization effect of the diblock copolymer.