Aromatic polyformamidines were synthesized by high temperature solution polycondensation of aromatic diamines with triethyl orthoformate. In order to obtain high molecular weight polymers, general investigations on the influence of reaction conditions such as reaction time, temperature and solvent were carried out on the basis of polyformamidine prepared from 4,4’-methylenedianiline. The proposed structures of aromatic polyformamidines were confirmed by elemental analysis, NMR- and IR-spectroscopy. Molecular weights were determined by H-1 NMR end group analysis. Polyformamidines having inherent viscosities of 0.25 - 0.36 dl/g show excellent solubility in polar solvents and in strong acids. Thermal analysis indicated that these polymers are stable up to 300-degrees-C, and a 10% weight loss were recorded on the TG curves in the temperature range of 350 - 366-degrees-C in nitrogen.