Homopolymer poly(ethylene terephthalate) (PET) and nylon-6 (PA6) and a blend (1:1 by weight) of these polymers, were extruded as strips and ultraquenched from the melt. After zone drawing and additional annealing at temperatures, T-a, of 220 or 240 degrees C for 5 or 25 h in vacuum, the samples were studied by scanning electron microscopy (SEM), wide-angle X-ray scattering, solubility and microhardness, H,tests. In conformity with previous studies of the same system, the present SEM observations show that mechanical drawing results in the formation of a highly oriented fibrillar structure of PET which is preserved even after annealing above the melting point of PA6. Furthermore, raising of both annealing temperature and duration up to 240 degrees C and 25 h, respectively, results in a strong decrease of the solubility of the PA6 fraction in formic acid (five-fold). This is attributed to intensive chemical interactions between components, drastically improving the adhesion between matrix and reinforcing microfibrils. From the dependence of H on degree of crystallinity, w(c), the hardness values for completely amorphous, H-a, and fully crystalline, H-c, neat omopolymers were extrapolated (H-a(PET) = 128 MPa, H-c(PET) = 294 MPa, H-a(PA) = 52 MPa and H-c(PA) = 283 MPa). Using these values and applying the additive law, the H-value of the microfibrils is derived. The high value obtained for PET fibrils (360 MPa) is explained by the peculiarity in the structure formation of these microfibrils. The effect of crystal size on the formation of H is also discussed. The H-value of infinite large PA6 crystals is derived to be H-infinity = 460 MPa. It is shown that the type and extent of the mutual dispersion of the components, as well as the adhesion between them, are important factors for the proper applicability of the additive law.