A poly(ether imide) (Ultem), which is an amorphous engineering thermoplastic, was melt blended with two liquid crystalline polymers : one based on p-hydroxybenzoic acid and 2-hydroxy-6-naphthoic acid (Vectra A900) and the other based on phenylhydroquinone, ethylphenylhydroquinone and terephthalic acid (Granlar). The miscibility of these two blend systems was studied by means of differential scanning calorimetry (d.s.c.), dynamic mechanical thermal analysis (d.m.t.a.), scanning electron microscopy (s.e.m.) and wide-angle X-ray scattering (WAXS). It was found that the Ultem/Vectra system is immiscible, while the Ultem/Granlar system is partially miscible. As revealed by d.m.t.a., the partial miscibility of the latter can be improved by increasing the degree of mixing in a single-screw extruder. The flexural and tensile moduli of the in situ reinforced composites based on these two blend systems were determined as a function of blend composition. For similar mixing conditions it was found that the flexural moduli of the Ultem/Granlar system showed a positive deviation from the simple rule of mixtures while those of the Ultem/Vectra system showed a negative deviation. The tensile moduli of the Ultem/Vectra system also showed a pronounced positive deviation only for blends with high Vectra compositions. These results are interpreted in terms of the differences in miscibility.