The effect of X-ray irradiation on crystallization of the blend of poly(N-methyldodecano-12-lactam) (PMDL) with 29.5% of statistical copolymer poly(styrene-stat-acrylic acid) (PSAA) was studied using DSC, WAXD, and solid-state C-13 NMR methods. Significant acceleration of crystallization was found in the as-prepared X-ray irradiated (XI), that is, WAXD-measured, samples compared with the nonirradiated (NI) ones. In the XI blend, the incubation period was shortened and crystallization proceeded at significantly higher rate. In the asymptote, after 100-120 days, both NI and XI samples reached the same final crystallinity of about 18%. The second DSC runs indicated that the stimulating effect of X-ray irradiation was eliminated by heating the sample during the first run. The C-13 NMR studies have shown that PMDL chains crystallize exclusively in cis conformation on the C-N bond. Both in neat PMDL and in the blend with PSAA, the XI samples contained a significantly higher proportion of cis conformers in amorphous phase than the NI samples. It is suggested that energy absorbed by the sample during the standard WAXD measurement helps overcome the barrier of the trans/cis transition in the PMDL molecules. This opens the way to the formation of a higher number of critical equilibrium nuclei and, finally, results in accelerated crystallization of the XI samples. No irreversible changes were found in the XI samples. (C) 2007 Wiley Periodicals, Inc.