Polymers with amidine groups -NH-CR=N- in the main chain were prepared by acid catalysed melt polycondensation of 4,4＇-diaminodiphenyl methane (DAPM) with various orthoesters. The resulting polyamidines were characterized by H-1-, C-13- and N-15-NMR spectroscopy, differential scanning calorimetry(DSC), thermogravimetry( TGA) and wide angle x-ray scattering(WAXS) measurements. The proper ties of the polyamidines are strongly determined by the substituent R on the amidine group. Some peculiarities of the polyformamidine (R=H) could be attributed to the conformation of the amidine group. Glass transition temperatures varied from 106 to 161 degrees C depending on the substituent R. All polyamidines possessed good thermal stability up to 370 degrees C and, with exception of the polyformamidine, good solubility in common solvents.