A series of erucamide (13-cis-docosenamide) and poly(laurolactam) (PA-12) blends have been studied by means of differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS) and dynamic mechanical thermal analysis (DMTA). The blends presented three very well differentiated types of behaviour to temperature depending on the erucamide content. Thus, for the lower erucamide content blends (0-6%), they show a good compatibility. For these samples, erucamide (ERU III) is dissolved in the amorphous regions of PA-12, which now may be identified as "melt-like or liquid-like amorphous component", decreasing the alpha-relaxation temperature (T-g) Of PA-12. In this case the additive acts as a plasticiser. For blends with an erucamide content higher than about 6 wt%, erucamide (ERU II) crystallises in the amorphous regions of PA-12, which have a segmental mobility in between those of the crystalline and the liquid-like amorphous component, as proved by the low-temperature endotherm that appears in the DSC traces. This may he a "disordered phase of anisotropic nature". Both types of amorphous regions can be considered as interphase components with similar characteristics to the liquid-like and crystalline components, respectively. They act as two true and different matrices with respect to the additive: one as a miscible and homogeneous mixture and the other as crystals dispersed in an amorphous phase. And for blends with still higher erucamide content, a part is segregated and crystallised as pure erucamide (ERU I) in the whole polymer matrix as microdomains or droplets.