New rigid-flexible copolymers based on poly(p-benzamide) (PBA) and poly[2-phenylene (5-benzimidazole)] (PABI) were synthesized, varying the molecular weight of the flexible block. Ternary solutions in dimethylacetamide-3% LiCl show the peculiar behaviour expected theoretically for this kind of system. In particular, it was demonstrated that more copolymer enters the anisotropic phase as the PABI chain length becomes shorter. Comparison with homopolymer mixtures shows the usefulness of the copolymer in improving the amount of flexible chains inside the mesophase.