4 pags. 2 figs. The microhardness, H, of random copolymers of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalene-2,6-dicarboxilate) (PEN) was determined over a wide range of compositions. It is shown that microhardness of the materials is strongly affected by the composition. The mechanical property, H, of the quenched amorphous copolyester films is discussed in terms of a simple model given by the additivity values of the single components Ha PET and Ha PEN. In materials containing up to 30% PEN, crystals of PET are found after annealing at temperatures 10 °C below their melting points. In materials containing ∼ 80% PEN, after annealing at about 20 °C below the melting point, crystals of PEN are formed. The observed deviation of H for the crystallized films from the additive behaviour of the single components can be quantitatively related to two factors: the changes occurring in the crystallinity value and in the thickness of PET and PEN crystals. © 1992 Chapman & Hall.