A diblock copolymer based on poly(p-benzamide) (PBA) and poly(2-phenylene-(5-benzimidazole))isophthalamide (PABI) was synthesized by direct coupling reaction between acylchloride and amino end groups of the two homopolymers. Soxhlet extraction with N,N’-dimethylacetamide completely removed unreacted PABI and allowed determination of copolymer content and composition of the unextracted fraction (by u.v. data and viscosity measurements). Preliminary results on phase behaviour are given and analysed in terms of block length.