This paper addresses a number of questions regarding the use of thermotropic liquid crystalline polymers (TLCPs) to reinforce thermoplastics. In particular, we concentrate on the effect of partial miscibility between the matrix and the TLCP, the effect of the properties exhibited by the TLCP itself, and why the properties of the blends pass through a maximum at an intermediate composition. Blends of an immiscible and partially miscible TLCP with a poly(ether imide) (PEI) were injection-moulded into mini-tensile bars and rectangular plaques, and their mechanical properties were evaluated. Interfacial and rheological properties along with morphological and molecular orientation analyses were carried out in order to explain the mechanical properties of the blends. Mechanical tests showed that both the tensile and flexural moduli deviate positively from the law of mixtures and, in addition, both blends showed a maximum in the tensile modulus at 90 wt% TLCP. This is believed to be due to a higher degree of molecular orientation observed for the blends than for the neat TLCPs. It was found that partial miscibility affected more strongly the stiffness of the in situ composite, whereas the ultimate properties of the TLCP dominated the ultimate properties of the PEI/TLCP composite.