Poly-p-phenylenetherephthalamide (PpPTA) is a high performance, semi flexible polymer. Commercially it is mainly sold in the form of fibres under the trade names Twaron and Kevlar. The polymerization is performed in an amide(NMP)-salt(CaCl2) solution and afterwards spun into fibres from a sulphuric acid solution. In the thesis the dissolution behaviour of PpPTA was investigated. It was found that PpPTA in NMP-CaCl2 , forms a meta-stable solution that phase separates. Other specific amide-salt systems can however, in contrast to generally believed, act as a thermodynamic stable solvent for PpPTA. The advantage lies in the possible development of a polymerization medium from which also fibre spinning can be done. A thermodynamic model was developed to predict the solvent power of PpPTA for a given mixture. Molecular modelling techniques were used to visualize the molecular interactions between PpPTA and its environment. Electrical charge was found to be essential in keeping the polymer dissolved. Furthermore evidence was found for the existence of so called “hairpin” formation in PpPTA chains. The polymer chain can fold back on itself at relative low energy cost. This makes the polymer less rigid than often assumed.