To reveal the effect of the nigrosine dye, that the addition of the dye lowers the crystallization point (T-c) of molten polyamide resins with substantially no shift in the melting point (T-m), thus suppressing the crystallization enhancement of the crystalline nucleation agents, the characteristics of polyamide 66 (PA-66) containing nigrosine dye EX (N-EX) were investigated. Differential scanning calorimetry (DSC) analysis showed that the addition of N-EX reduced the crystallization rate and T-c of molten PA-66 with substantially no shift in T-m. and the crystallization enthalpy per unit of weight of PA-66 was substantially constant. T-c of molten PA-66 was lowered with an increase in the amount of N-EX and reached its maximum at 13 wt % N-EX. Dynamic mechanical analysis showed that the glass-transition temperature and the secondary glass-transition temperature increased with an increasing amount of the dye. On the other hand, the DSC and X-ray diffraction results indicated that no dye molecule was present in the crystal structure of PA-66. This effect of the nigrosine dye on PA-66 is in contrast to those of crystalline nucleation agents, plasticizers, and antiplasticizers. (c) 2006 Wiley Periodicals, Inc.